diff --git a/platform_include/nativehelper/detail/signature_checker.h b/platform_include/nativehelper/detail/signature_checker.h
new file mode 100644
index 0000000..b4ea68e
--- /dev/null
+++ b/platform_include/nativehelper/detail/signature_checker.h
@@ -0,0 +1,1437 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+
+/*
+ * WARNING: Do not include and use these directly. Use jni_macros.h instead!
+ * The "detail" namespace should be a strong hint not to depend on the internals,
+ * which could change at any time.
+ *
+ * This implements the underlying mechanism for compile-time JNI signature/ctype checking
+ * and inference.
+ *
+ * This file provides the constexpr basic blocks such as strings, arrays, vectors
+ * as well as the JNI-specific parsing functionality.
+ *
+ * Everything is implemented via generic-style (templates without metaprogramming)
+ * wherever possible. Traditional template metaprogramming is used sparingly.
+ *
+ * Everything in this file except ostream<< is constexpr.
+ */
+
+#pragma once
+
+#include <iostream>     // std::ostream
+#include <jni.h>        // jni typedefs, JniNativeMethod.
+#include <type_traits>  // std::common_type, std::remove_cv
+
+namespace nativehelper {
+namespace detail {
+
+// If CHECK evaluates to false then X_ASSERT will halt compilation.
+//
+// Asserts meant to be used only within constexpr context.
+#if defined(JNI_SIGNATURE_CHECKER_DISABLE_ASSERTS)
+# define X_ASSERT(CHECK) do { if ((false)) { (CHECK) ? void(0) : void(0); } } while (false)
+#else
+# define X_ASSERT(CHECK) \
+    ( (CHECK) ? void(0) : jni_assertion_failure(#CHECK) )
+#endif
+
+// The runtime 'jni_assert' will never get called from a constexpr context;
+// instead compilation will abort with a stack trace.
+//
+// Inspect the frame above this one to see the exact nature of the failure.
+inline void jni_assertion_failure(const char* /*msg*/) __attribute__((noreturn));
+inline void jni_assertion_failure(const char* /*msg*/) {
+  std::terminate();
+}
+
+// An immutable constexpr string view, similar to std::string_view but for C++14.
+// For a mutable string see instead ConstexprVector<char>.
+//
+// As it is a read-only view into a string, it is not guaranteed to be zero-terminated.
+struct ConstexprStringView {
+  // Implicit conversion from string literal:
+  //     ConstexprStringView str = "hello_world";
+  template<size_t N>
+  constexpr ConstexprStringView(const char (& lit)[N])  // NOLINT: explicit.
+      : _array(lit), _size(N - 1) {
+    // Using an array of characters is not allowed because the inferred size would be wrong.
+    // Use the other constructor instead for that.
+    X_ASSERT(lit[N - 1] == '\0');
+  }
+
+  constexpr ConstexprStringView(const char* ptr, size_t size)
+      : _array(ptr), _size(size) {
+    // See the below constructor instead.
+    X_ASSERT(ptr != nullptr);
+  }
+
+  // Implicit conversion from nullptr, creates empty view.
+  //   ConstexprStringView str = nullptr;
+  explicit constexpr ConstexprStringView(const decltype(nullptr)&)
+      : _array(""), _size(0u) {
+  }
+
+  // No-arg constructor: Create empty view.
+  constexpr ConstexprStringView() : _array(""), _size(0u) {}
+
+  constexpr size_t size() const {
+    return _size;
+  }
+
+  constexpr bool empty() const {
+    return size() == 0u;
+  }
+
+  constexpr char operator[](size_t i) const {
+    X_ASSERT(i <= size());
+    return _array[i];
+  }
+
+  // Create substring from this[start..start+len).
+  constexpr ConstexprStringView substr(size_t start, size_t len) const {
+    X_ASSERT(start <= size());
+    X_ASSERT(start + len <= size());
+
+    return ConstexprStringView(&_array[start], len);
+  }
+
+  // Create maximum length substring that begins at 'start'.
+  constexpr ConstexprStringView substr(size_t start) const {
+    X_ASSERT(start <= size());
+    return substr(start, size() - start);
+  }
+
+  using const_iterator = const char*;
+
+  constexpr const_iterator begin() const {
+    return &_array[0];
+  }
+
+  constexpr const_iterator end() const {
+    return &_array[size()];
+  }
+
+ private:
+  const char* _array;  // Never-null for simplicity.
+  size_t _size;
+};
+
+constexpr bool
+operator==(const ConstexprStringView& lhs, const ConstexprStringView& rhs) {
+  if (lhs.size() != rhs.size()) {
+    return false;
+  }
+  for (size_t i = 0; i < lhs.size(); ++i) {
+    if (lhs[i] != rhs[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
+constexpr bool
+operator!=(const ConstexprStringView& lhs, const ConstexprStringView& rhs) {
+  return !(lhs == rhs);
+}
+
+inline std::ostream& operator<<(std::ostream& os, const ConstexprStringView& str) {
+  for (char c : str) {
+    os << c;
+  }
+  return os;
+}
+
+constexpr bool IsValidJniDescriptorShorty(char shorty) {
+  constexpr char kValidJniTypes[] =
+      {'V', 'Z', 'B', 'C', 'S', 'I', 'J', 'F', 'D', 'L', '[', '(', ')'};
+
+  for (char c : kValidJniTypes) {
+    if (c == shorty) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+// A constexpr "vector" that supports storing a variable amount of Ts
+// in an array-like interface.
+//
+// An up-front kMaxSize must be given since constexpr does not support
+// dynamic allocations.
+template<typename T, size_t kMaxSize>
+struct ConstexprVector {
+ public:
+  constexpr explicit ConstexprVector() : _size(0u), _array{} {
+  }
+
+ private:
+  // Custom iterator to support ptr-one-past-end into the union array without
+  // undefined behavior.
+  template<typename Elem>
+  struct VectorIterator {
+    Elem* ptr;
+
+    constexpr VectorIterator& operator++() {
+      ++ptr;
+      return *this;
+    }
+
+    constexpr VectorIterator operator++(int) const {
+      VectorIterator tmp(*this);
+      ++tmp;
+      return tmp;
+    }
+
+    constexpr auto& operator*() {
+      // Use 'auto' here since using 'T' is incorrect with const_iterator.
+      return ptr->_value;
+    }
+
+    constexpr const T& operator*() const {
+      return ptr->_value;
+    }
+
+    constexpr bool operator==(const VectorIterator& other) const {
+      return ptr == other.ptr;
+    }
+
+    constexpr bool operator!=(const VectorIterator& other) const {
+      return !(*this == other);
+    }
+  };
+
+  // Do not require that T is default-constructible by using a union.
+  struct MaybeElement {
+    union {
+      T _value;
+    };
+  };
+
+ public:
+  using iterator = VectorIterator<MaybeElement>;
+  using const_iterator = VectorIterator<const MaybeElement>;
+
+  constexpr iterator begin() {
+    return {&_array[0]};
+  }
+
+  constexpr iterator end() {
+    return {&_array[size()]};
+  }
+
+  constexpr const_iterator begin() const {
+    return {&_array[0]};
+  }
+
+  constexpr const_iterator end() const {
+    return {&_array[size()]};
+  }
+
+  constexpr void push_back(const T& value) {
+    X_ASSERT(_size + 1 <= kMaxSize);
+
+    _array[_size]._value = value;
+    _size++;
+  }
+
+  // A pop operation could also be added since constexpr T's
+  // have default destructors, it would just be _size--.
+  // We do not need a pop() here though.
+
+  constexpr const T& operator[](size_t i) const {
+    return _array[i]._value;
+  }
+
+  constexpr T& operator[](size_t i) {
+    return _array[i]._value;
+  }
+
+  constexpr size_t size() const {
+    return _size;
+  }
+ private:
+
+  size_t _size;
+  MaybeElement _array[kMaxSize];
+};
+
+// Parsed and validated "long" form of a single JNI descriptor.
+// e.g. one of "J", "Ljava/lang/Object;" etc.
+struct JniDescriptorNode {
+  ConstexprStringView longy;
+
+  constexpr JniDescriptorNode(ConstexprStringView longy)
+      : longy(longy) {  // NOLINT: explicit.
+    X_ASSERT(!longy.empty());
+  }
+  constexpr JniDescriptorNode() : longy() {}
+
+  constexpr char shorty() {
+    // Must be initialized with the non-default constructor.
+    X_ASSERT(!longy.empty());
+    return longy[0];
+  }
+};
+
+inline std::ostream& operator<<(std::ostream& os, const JniDescriptorNode& node) {
+  os << node.longy;
+  return os;
+}
+
+// Equivalent of C++17 std::optional.
+//
+// An optional is essentially a type safe
+//    union {
+//      void Nothing,
+//      T    Some;
+//    };
+//
+template<typename T>
+struct ConstexprOptional {
+  // Create a default optional with no value.
+  constexpr ConstexprOptional() : _has_value(false), _nothing() {
+  }
+
+  // Create an optional with a value.
+  constexpr ConstexprOptional(const T& value)
+      : _has_value(true), _value(value) {
+  }
+
+  constexpr explicit operator bool() const {
+    return _has_value;
+  }
+
+  constexpr bool has_value() const {
+    return _has_value;
+  }
+
+  constexpr const T& value() const {
+    X_ASSERT(has_value());
+    return _value;
+  }
+
+  constexpr const T* operator->() const {
+    return &(value());
+  }
+
+ private:
+  bool _has_value;
+  // The "Nothing" is likely unnecessary but improves readability.
+  struct Nothing {};
+  union {
+    Nothing _nothing;
+    T _value;
+  };
+};
+
+template<typename T>
+constexpr bool
+operator==(const ConstexprOptional<T>& lhs, const ConstexprOptional<T>& rhs) {
+  if (lhs && rhs) {
+    return lhs.value() == rhs.value();
+  }
+  return lhs.has_value() == rhs.has_value();
+}
+
+template<typename T>
+constexpr bool
+operator!=(const ConstexprOptional<T>& lhs, const ConstexprOptional<T>& rhs) {
+  return !(lhs == rhs);
+}
+
+template<typename T>
+inline std::ostream& operator<<(std::ostream& os, const ConstexprOptional<T>& val) {
+  if (val) {
+    os << val.value();
+  }
+  return os;
+}
+
+// Equivalent of std::nullopt
+// Allows implicit conversion to any empty ConstexprOptional<T>.
+// Mostly useful for macros that need to return an empty constexpr optional.
+struct NullConstexprOptional {
+  template<typename T>
+  constexpr operator ConstexprOptional<T>() const {
+    return ConstexprOptional<T>();
+  }
+};
+
+inline std::ostream& operator<<(std::ostream& os, NullConstexprOptional) {
+  return os;
+}
+
+#if !defined(PARSE_FAILURES_NONFATAL)
+// Unfortunately we cannot have custom messages here, as it just prints a stack trace with the macros expanded.
+// This is at least more flexible than static_assert which requires a string literal.
+// NOTE: The message string literal must be on same line as the macro to be seen during a compilation error.
+#define PARSE_FAILURE(msg) X_ASSERT(! #msg)
+#define PARSE_ASSERT_MSG(cond, msg) X_ASSERT(#msg && (cond))
+#define PARSE_ASSERT(cond) X_ASSERT(cond)
+#else
+#define PARSE_FAILURE(msg) return NullConstexprOptional{};
+#define PARSE_ASSERT_MSG(cond, msg) if (!(cond)) { PARSE_FAILURE(msg); }
+#define PARSE_ASSERT(cond) if (!(cond)) { PARSE_FAILURE(""); }
+#endif
+
+// This is a placeholder function and should not be called directly.
+constexpr void ParseFailure(const char* msg) {
+  (void) msg;  // intentionally no-op.
+}
+
+// Temporary parse data when parsing a function descriptor.
+struct ParseTypeDescriptorResult {
+  // A single argument descriptor, e.g. "V" or "Ljava/lang/Object;"
+  ConstexprStringView token;
+  // The remainder of the function descriptor yet to be parsed.
+  ConstexprStringView remainder;
+
+  constexpr bool has_token() const {
+    return token.size() > 0u;
+  }
+
+  constexpr bool has_remainder() const {
+    return remainder.size() > 0u;
+  }
+
+  constexpr JniDescriptorNode as_node() const {
+    X_ASSERT(has_token());
+    return {token};
+  }
+};
+
+// Parse a single type descriptor out of a function type descriptor substring,
+// and return the token and the remainder string.
+//
+// If parsing fails (i.e. illegal syntax), then:
+//    parses are fatal -> assertion is triggered (default behavior),
+//    parses are nonfatal -> returns nullopt (test behavior).
+constexpr ConstexprOptional<ParseTypeDescriptorResult>
+ParseSingleTypeDescriptor(ConstexprStringView single_type,
+                          bool allow_void = false) {
+  constexpr NullConstexprOptional kUnreachable = {};
+
+  // Nothing else left.
+  if (single_type.size() == 0) {
+    return ParseTypeDescriptorResult{};
+  }
+
+  ConstexprStringView token;
+  ConstexprStringView remainder = single_type.substr(/*start*/1u);
+
+  char c = single_type[0];
+  PARSE_ASSERT(IsValidJniDescriptorShorty(c));
+
+  enum State {
+    kSingleCharacter,
+    kArray,
+    kObject
+  };
+
+  State state = kSingleCharacter;
+
+  // Parse the first character to figure out if we should parse the rest.
+  switch (c) {
+    case '!': {
+      constexpr bool fast_jni_is_deprecated = false;
+      PARSE_ASSERT(fast_jni_is_deprecated);
+      break;
+    }
+    case 'V':
+      if (!allow_void) {
+        constexpr bool void_type_descriptor_only_allowed_in_return_type = false;
+        PARSE_ASSERT(void_type_descriptor_only_allowed_in_return_type);
+      }
+      [[clang::fallthrough]];
+    case 'Z':
+    case 'B':
+    case 'C':
+    case 'S':
+    case 'I':
+    case 'J':
+    case 'F':
+    case 'D':
+      token = single_type.substr(/*start*/0u, /*len*/1u);
+      break;
+    case 'L':
+      state = kObject;
+      break;
+    case '[':
+      state = kArray;
+      break;
+    default: {
+      // See JNI Chapter 3: Type Signatures.
+      PARSE_FAILURE("Expected a valid type descriptor character.");
+      return kUnreachable;
+    }
+  }
+
+  // Possibly parse an arbitary-long remainder substring.
+  switch (state) {
+    case kSingleCharacter:
+      return {{token, remainder}};
+    case kArray: {
+      // Recursively parse the array component, as it's just any non-void type descriptor.
+      ConstexprOptional<ParseTypeDescriptorResult>
+          maybe_res = ParseSingleTypeDescriptor(remainder, /*allow_void*/false);
+      PARSE_ASSERT(maybe_res);  // Downstream parsing has asserted, bail out.
+
+      ParseTypeDescriptorResult res = maybe_res.value();
+
+      // Reject illegal array type descriptors such as "]".
+      PARSE_ASSERT_MSG(res.has_token(),
+                       "All array types must follow by their component type (e.g. ']I', ']]Z', etc. ");
+
+      token = single_type.substr(/*start*/0u, res.token.size() + 1u);
+
+      return {{token, res.remainder}};
+    }
+    case kObject: {
+      // Parse the fully qualified class, e.g. Lfoo/bar/baz;
+      // Note checking that each part of the class name is a valid class identifier
+      // is too complicated (JLS 3.8).
+      // This simple check simply scans until the next ';'.
+      bool found_semicolon = false;
+      size_t semicolon_len = 0;
+      for (size_t i = 0; i < single_type.size(); ++i) {
+        if (single_type[i] == ';') {
+          semicolon_len = i + 1;
+          found_semicolon = true;
+          break;
+        }
+      }
+
+      PARSE_ASSERT(found_semicolon);
+
+      token = single_type.substr(/*start*/0u, semicolon_len);
+      remainder = single_type.substr(/*start*/semicolon_len);
+
+      bool class_name_is_empty = token.size() <= 2u;  // e.g. "L;"
+      PARSE_ASSERT(!class_name_is_empty);
+
+      return {{token, remainder}};
+    }
+    default:
+      X_ASSERT(false);
+  }
+
+  X_ASSERT(false);
+  return kUnreachable;
+}
+
+// Abstract data type to represent container for Ret(Args,...).
+template<typename T, size_t kMaxSize>
+struct FunctionSignatureDescriptor {
+  ConstexprVector<T, kMaxSize> args;
+  T ret;
+
+  static constexpr size_t max_size = kMaxSize;
+};
+
+
+template<typename T, size_t kMaxSize>
+inline std::ostream& operator<<(std::ostream& os,
+                                const FunctionSignatureDescriptor<T,
+                                                                  kMaxSize>& signature) {
+  size_t count = 0;
+  os << "args={";
+  for (auto& arg : signature.args) {
+    os << arg;
+
+    if (count != signature.args.size() - 1) {
+      os << ",";
+    }
+
+    ++count;
+  }
+  os << "}, ret=";
+  os << signature.ret;
+  return os;
+}
+
+// Ret(Args...) of JniDescriptorNode.
+template<size_t kMaxSize>
+using JniSignatureDescriptor = FunctionSignatureDescriptor<JniDescriptorNode,
+                                                           kMaxSize>;
+
+// Parse a JNI function signature descriptor into a JniSignatureDescriptor.
+//
+// If parsing fails (i.e. illegal syntax), then:
+//    parses are fatal -> assertion is triggered (default behavior),
+//    parses are nonfatal -> returns nullopt (test behavior).
+template<size_t kMaxSize>
+constexpr ConstexprOptional<JniSignatureDescriptor<kMaxSize>>
+ParseSignatureAsList(ConstexprStringView signature) {
+  // The list of JNI descritors cannot possibly exceed the number of characters
+  // in the JNI string literal. We leverage this to give an upper bound of the strlen.
+  // This is a bit wasteful but in constexpr there *must* be a fixed upper size for data structures.
+  ConstexprVector<JniDescriptorNode, kMaxSize> jni_desc_node_list;
+  JniDescriptorNode return_jni_desc;
+
+  enum State {
+    kInitial = 0,
+    kParsingParameters = 1,
+    kParsingReturnType = 2,
+    kCompleted = 3,
+  };
+
+  State state = kInitial;
+
+  while (!signature.empty()) {
+    switch (state) {
+      case kInitial: {
+        char c = signature[0];
+        PARSE_ASSERT_MSG(c == '(',
+                         "First character of a JNI signature must be a '('");
+        state = kParsingParameters;
+        signature = signature.substr(/*start*/1u);
+        break;
+      }
+      case kParsingParameters: {
+        char c = signature[0];
+        if (c == ')') {
+          state = kParsingReturnType;
+          signature = signature.substr(/*start*/1u);
+          break;
+        }
+
+        ConstexprOptional<ParseTypeDescriptorResult>
+            res = ParseSingleTypeDescriptor(signature, /*allow_void*/false);
+        PARSE_ASSERT(res);
+
+        jni_desc_node_list.push_back(res->as_node());
+
+        signature = res->remainder;
+        break;
+      }
+      case kParsingReturnType: {
+        ConstexprOptional<ParseTypeDescriptorResult>
+            res = ParseSingleTypeDescriptor(signature, /*allow_void*/true);
+        PARSE_ASSERT(res);
+
+        return_jni_desc = res->as_node();
+        signature = res->remainder;
+        state = kCompleted;
+        break;
+      }
+      default: {
+        // e.g. "()VI" is illegal because the V terminates the signature.
+        PARSE_FAILURE("Signature had left over tokens after parsing return type");
+        break;
+      }
+    }
+  }
+
+  switch (state) {
+    case kCompleted:
+      // Everything is ok.
+      break;
+    case kParsingParameters:
+      PARSE_FAILURE("Signature was missing ')'");
+      break;
+    case kParsingReturnType:
+      PARSE_FAILURE("Missing return type");
+    case kInitial:
+      PARSE_FAILURE("Cannot have an empty signature");
+    default:
+      X_ASSERT(false);
+  }
+
+  return {{jni_desc_node_list, return_jni_desc}};
+}
+
+// What kind of JNI does this type belong to?
+enum NativeKind {
+  kNotJni,        // Illegal parameter used inside of a function type.
+  kNormalJniCallingConventionParameter,
+  kNormalNative,
+  kFastNative,      // Also valid in normal.
+  kCriticalNative,  // Also valid in fast/normal.
+};
+
+// Is this type final, i.e. it cannot be subtyped?
+enum TypeFinal {
+  kNotFinal,
+  kFinal         // e.g. any primitive or any "final" class such as String.
+};
+
+// What position is the JNI type allowed to be in?
+// Ignored when in a CriticalNative context.
+enum NativePositionAllowed {
+  kNotAnyPosition,
+  kReturnPosition,
+  kZerothPosition,
+  kFirstOrLaterPosition,
+  kSecondOrLaterPosition,
+};
+
+constexpr NativePositionAllowed ConvertPositionToAllowed(size_t position) {
+  switch (position) {
+    case 0:
+      return kZerothPosition;
+    case 1:
+      return kFirstOrLaterPosition;
+    default:
+      return kSecondOrLaterPosition;
+  }
+}
+
+// Type traits for a JNI parameter type. See below for specializations.
+template<typename T>
+struct jni_type_trait {
+  static constexpr NativeKind native_kind = kNotJni;
+  static constexpr const char type_descriptor[] = "(illegal)";
+  static constexpr NativePositionAllowed position_allowed = kNotAnyPosition;
+  static constexpr TypeFinal type_finality = kNotFinal;
+  static constexpr const char type_name[] = "(illegal)";
+};
+
+// Access the jni_type_trait<T> from a non-templated constexpr function.
+// Identical non-static fields to jni_type_trait, see Reify().
+struct ReifiedJniTypeTrait {
+  NativeKind native_kind;
+  ConstexprStringView type_descriptor;
+  NativePositionAllowed position_allowed;
+  TypeFinal type_finality;
+  ConstexprStringView type_name;
+
+  template<typename T>
+  static constexpr ReifiedJniTypeTrait Reify() {
+    // This should perhaps be called 'Type Erasure' except we don't use virtuals,
+    // so it's not quite the same idiom.
+    using TR = jni_type_trait<T>;
+    return {TR::native_kind,
+            TR::type_descriptor,
+            TR::position_allowed,
+            TR::type_finality,
+            TR::type_name};
+  }
+
+  // Find the most similar ReifiedJniTypeTrait corresponding to the type descriptor.
+  //
+  // Any type can be found by using the exact canonical type descriptor as listed
+  // in the jni type traits definitions.
+  //
+  // Non-final JNI types have limited support for inexact similarity:
+  //   [[* | [L* -> jobjectArray
+  //   L* -> jobject
+  //
+  // Otherwise return a nullopt.
+  static constexpr ConstexprOptional<ReifiedJniTypeTrait>
+  MostSimilarTypeDescriptor(ConstexprStringView type_descriptor);
+};
+
+constexpr bool
+operator==(const ReifiedJniTypeTrait& lhs, const ReifiedJniTypeTrait& rhs) {
+  return lhs.native_kind == rhs.native_kind
+      && rhs.type_descriptor == lhs.type_descriptor &&
+      lhs.position_allowed == rhs.position_allowed
+      && rhs.type_finality == lhs.type_finality &&
+      lhs.type_name == rhs.type_name;
+}
+
+inline std::ostream& operator<<(std::ostream& os, const ReifiedJniTypeTrait& rjft) {
+  // os << "ReifiedJniTypeTrait<" << rjft.type_name << ">";
+  os << rjft.type_name;
+  return os;
+}
+
+// Template specialization for any JNI typedefs.
+#define JNI_TYPE_TRAIT(jtype, the_type_descriptor, the_native_kind, the_type_finality, the_position) \
+template <>                                                                    \
+struct jni_type_trait< jtype > {                                               \
+  static constexpr NativeKind native_kind = the_native_kind;                   \
+  static constexpr const char type_descriptor[] = the_type_descriptor;         \
+  static constexpr NativePositionAllowed position_allowed = the_position;      \
+  static constexpr TypeFinal type_finality = the_type_finality;                \
+  static constexpr const char type_name[] = #jtype;                            \
+};
+
+#define DEFINE_JNI_TYPE_TRAIT(TYPE_TRAIT_FN)                                                                  \
+TYPE_TRAIT_FN(jboolean,          "Z",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jbyte,             "B",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jchar,             "C",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jshort,            "S",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jint,              "I",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jlong,             "J",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jfloat,            "F",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jdouble,           "D",                      kCriticalNative,   kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jobject,           "Ljava/lang/Object;",     kFastNative,    kNotFinal, kFirstOrLaterPosition)  \
+TYPE_TRAIT_FN(jclass,            "Ljava/lang/Class;",      kFastNative,       kFinal, kFirstOrLaterPosition)  \
+TYPE_TRAIT_FN(jstring,           "Ljava/lang/String;",     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jarray,            "Ljava/lang/Object;",     kFastNative,    kNotFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jobjectArray,      "[Ljava/lang/Object;",    kFastNative,    kNotFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jbooleanArray,     "[Z",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jbyteArray,        "[B",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jcharArray,        "[C",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jshortArray,       "[S",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jintArray,         "[I",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jlongArray,        "[J",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jfloatArray,       "[F",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jdoubleArray,      "[D",                     kFastNative,       kFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(jthrowable,        "Ljava/lang/Throwable;",  kFastNative,    kNotFinal, kSecondOrLaterPosition) \
+TYPE_TRAIT_FN(JNIEnv*,           "",                       kNormalJniCallingConventionParameter, kFinal, kZerothPosition) \
+TYPE_TRAIT_FN(void,              "V",                      kCriticalNative,   kFinal, kReturnPosition)        \
+
+DEFINE_JNI_TYPE_TRAIT(JNI_TYPE_TRAIT)
+
+// See ReifiedJniTypeTrait for documentation.
+constexpr ConstexprOptional<ReifiedJniTypeTrait>
+ReifiedJniTypeTrait::MostSimilarTypeDescriptor(ConstexprStringView type_descriptor) {
+#define MATCH_EXACT_TYPE_DESCRIPTOR_FN(type, type_desc, native_kind, ...) \
+    if (type_descriptor == type_desc && native_kind >= kNormalNative) {                        \
+      return { Reify<type>() };                                \
+    }
+
+  // Attempt to look up by the precise type match first.
+  DEFINE_JNI_TYPE_TRAIT(MATCH_EXACT_TYPE_DESCRIPTOR_FN);
+
+  // Otherwise, we need to do an imprecise match:
+  char shorty = type_descriptor.size() >= 1 ? type_descriptor[0] : '\0';
+  if (shorty == 'L') {
+    // Something more specific like Ljava/lang/Throwable, string, etc
+    // is already matched by the macro-expanded conditions above.
+    return {Reify<jobject>()};
+  } else if (type_descriptor.size() >= 2) {
+    auto shorty_shorty = type_descriptor.substr(/*start*/0, /*size*/2u);
+    if (shorty_shorty == "[[" || shorty_shorty == "[L") {
+      // JNI arrays are covariant, so any type T[] (T!=primitive) is castable to Object[].
+      return {Reify<jobjectArray>()};
+    }
+  }
+
+  // To handle completely invalid values.
+  return NullConstexprOptional{};
+}
+
+// Check if a jni parameter type is valid given its position and native_kind.
+template <typename T>
+constexpr bool IsValidJniParameter(NativeKind native_kind, NativePositionAllowed position) {
+  // const,volatile does not affect JNI compatibility since it does not change ABI.
+  using expected_trait = jni_type_trait<typename std::remove_cv<T>::type>;
+  NativeKind expected_native_kind = expected_trait::native_kind;
+
+  // Most types 'T' are not valid for JNI.
+  if (expected_native_kind == NativeKind::kNotJni) {
+    return false;
+  }
+
+  // The rest of the types might be valid, but it depends on the context (native_kind)
+  // and also on their position within the parameters.
+
+  // Position-check first. CriticalNatives ignore positions since the first 2 special parameters are stripped.
+  while (native_kind != kCriticalNative) {
+    NativePositionAllowed expected_position = expected_trait::position_allowed;
+    X_ASSERT(expected_position != kNotAnyPosition);
+
+    // Is this a return-only position?
+    if (expected_position == kReturnPosition) {
+      if (position != kReturnPosition) {
+        // void can only be in the return position.
+        return false;
+      }
+      // Don't do the other non-return position checks for a return-only position.
+      break;
+    }
+
+    // JNIEnv* can only be in the first spot.
+    if (position == kZerothPosition && expected_position != kZerothPosition) {
+      return false;
+      // jobject, jclass can be 1st or anywhere afterwards.
+    } else if (position == kFirstOrLaterPosition
+        && expected_position != kFirstOrLaterPosition) {
+      return false;
+      // All other parameters must be in 2nd+ spot, or in the return type.
+    } else if (position == kSecondOrLaterPosition
+        || position == kReturnPosition) {
+      if (expected_position != kFirstOrLaterPosition
+          && expected_position != kSecondOrLaterPosition) {
+        return false;
+      }
+    }
+
+    break;
+  }
+
+  // Ensure the type appropriate is for the native kind.
+  if (expected_native_kind == kNormalJniCallingConventionParameter) {
+    // It's always wrong to use a JNIEnv* anywhere but the 0th spot.
+    if (native_kind == kCriticalNative) {
+      // CriticalNative does not allow using a JNIEnv*.
+      return false;
+    }
+
+    return true;  // OK: JniEnv* used in 0th position.
+  } else if (expected_native_kind == kCriticalNative) {
+    // CriticalNative arguments are always valid JNI types anywhere used.
+    return true;
+  } else if (native_kind == kCriticalNative) {
+    // The expected_native_kind was non-critical but we are in a critical context.
+    // Illegal type.
+    return false;
+  }
+
+  // Everything else is fine, e.g. fast/normal native + fast/normal native parameters.
+  return true;
+}
+
+// Is there sufficient number of parameters given the kind of JNI that it is?
+constexpr bool IsJniParameterCountValid(NativeKind native_kind, size_t count) {
+  if (native_kind == kNormalNative || native_kind == kFastNative) {
+    return count >= 2u;
+  } else if (native_kind == kCriticalNative) {
+    return true;
+  }
+
+  constexpr bool invalid_parameter = false;
+  X_ASSERT(invalid_parameter);
+  return false;
+}
+
+// Basic template interface. See below for partial specializations.
+//
+// Each instantiation will have a 'value' field that determines whether or not
+// all of the Args are valid JNI arguments given their native_kind.
+template<NativeKind native_kind, size_t position, typename ... Args>
+struct is_valid_jni_argument_type {
+  // static constexpr bool value = ?;
+};
+
+template<NativeKind native_kind, size_t position>
+struct is_valid_jni_argument_type<native_kind, position> {
+  static constexpr bool value = true;
+};
+
+template<NativeKind native_kind, size_t position, typename T>
+struct is_valid_jni_argument_type<native_kind, position, T> {
+  static constexpr bool value =
+      IsValidJniParameter<T>(native_kind, ConvertPositionToAllowed(position));
+};
+
+template<NativeKind native_kind, size_t position, typename T, typename ... Args>
+struct is_valid_jni_argument_type<native_kind, position, T, Args...> {
+  static constexpr bool value =
+      IsValidJniParameter<T>(native_kind, ConvertPositionToAllowed(position))
+          && is_valid_jni_argument_type<native_kind,
+                                        position + 1,
+                                        Args...>::value;
+};
+
+// This helper is required to decompose the function type into a list of arg types.
+template<NativeKind native_kind, typename T, T fn>
+struct is_valid_jni_function_type_helper;
+
+template<NativeKind native_kind, typename R, typename ... Args, R fn(Args...)>
+struct is_valid_jni_function_type_helper<native_kind, R(Args...), fn> {
+  static constexpr bool value =
+      IsJniParameterCountValid(native_kind, sizeof...(Args))
+          && IsValidJniParameter<R>(native_kind, kReturnPosition)
+          && is_valid_jni_argument_type<native_kind, /*position*/
+                                        0,
+                                        Args...>::value;
+};
+
+// Is this function type 'T' a valid C++ function type given the native_kind?
+template<NativeKind native_kind, typename T, T fn>
+constexpr bool IsValidJniFunctionType() {
+  return is_valid_jni_function_type_helper<native_kind, T, fn>::value;
+  // TODO: we could replace template metaprogramming with constexpr by
+  // using FunctionTypeMetafunction.
+}
+
+// Many parts of std::array is not constexpr until C++17.
+template<typename T, size_t N>
+struct ConstexprArray {
+  // Intentionally public to conform to std::array.
+  // This means all constructors are implicit.
+  // *NOT* meant to be used directly, use the below functions instead.
+  //
+  // The reason std::array has it is to support direct-list-initialization,
+  // e.g. "ConstexprArray<T, sz>{T{...}, T{...}, T{...}, ...};"
+  //
+  // Note that otherwise this would need a very complicated variadic
+  // argument constructor to only support list of Ts.
+  T _array[N];
+
+  constexpr size_t size() const {
+    return N;
+  }
+
+  using iterator = T*;
+  using const_iterator = const T*;
+
+  constexpr iterator begin() {
+    return &_array[0];
+  }
+
+  constexpr iterator end() {
+    return &_array[N];
+  }
+
+  constexpr const_iterator begin() const {
+    return &_array[0];
+  }
+
+  constexpr const_iterator end() const {
+    return &_array[N];
+  }
+
+  constexpr T& operator[](size_t i) {
+    return _array[i];
+  }
+
+  constexpr const T& operator[](size_t i) const {
+    return _array[i];
+  }
+};
+
+// Why do we need this?
+// auto x = {1,2,3} creates an initializer_list,
+//   but they can't be returned because it contains pointers to temporaries.
+// auto x[] = {1,2,3} doesn't even work because auto for arrays is not supported.
+//
+// an alternative would be to pull up std::common_t directly into the call site
+//   std::common_type_t<Args...> array[] = {1,2,3}
+// but that's even more cludgier.
+//
+// As the other "stdlib-wannabe" functions, it's weaker than the library
+// fundamentals std::make_array but good enough for our use.
+template<typename... Args>
+constexpr auto MakeArray(Args&& ... args) {
+  return ConstexprArray<typename std::common_type<Args...>::type,
+                        sizeof...(Args)>{args...};
+}
+
+// See below.
+template<typename T, T fn>
+struct FunctionTypeMetafunction {
+};
+
+// Enables the "map" operation over the function component types.
+template<typename R, typename ... Args, R fn(Args...)>
+struct FunctionTypeMetafunction<R(Args...), fn> {
+  // Count how many arguments there are, and add 1 for the return type.
+  static constexpr size_t
+      count = sizeof...(Args) + 1u;  // args and return type.
+
+  // Return an array where the metafunction 'Func' has been applied
+  // to every argument type. The metafunction must be returning a common type.
+  template<template<typename Arg> class Func>
+  static constexpr auto map_args() {
+    return map_args_impl<Func>(holder < Args > {}...);
+  }
+
+  // Apply the metafunction 'Func' over the return type.
+  template<template<typename Ret> class Func>
+  static constexpr auto map_return() {
+    return Func<R>{}();
+  }
+
+ private:
+  template<typename T>
+  struct holder {
+  };
+
+  template<template<typename Arg> class Func, typename Arg0, typename... ArgsRest>
+  static constexpr auto map_args_impl(holder<Arg0>, holder<ArgsRest>...) {
+    // One does not simply call MakeArray with 0 template arguments...
+    auto array = MakeArray(
+        Func<Args>{}()...
+    );
+
+    return array;
+  }
+
+  template<template<typename Arg> class Func>
+  static constexpr auto map_args_impl() {
+    // This overload provides support for MakeArray() with 0 arguments.
+    using ComponentType = decltype(Func<void>{}());
+
+    return ConstexprArray<ComponentType, /*size*/0u>{};
+  }
+};
+
+// Apply ReifiedJniTypeTrait::Reify<T> for every function component type.
+template<typename T>
+struct ReifyJniTypeMetafunction {
+  constexpr ReifiedJniTypeTrait operator()() const {
+    auto res = ReifiedJniTypeTrait::Reify<T>();
+    X_ASSERT(res.native_kind != kNotJni);
+    return res;
+  }
+};
+
+// Ret(Args...) where every component is a ReifiedJniTypeTrait.
+template<size_t kMaxSize>
+using ReifiedJniSignature = FunctionSignatureDescriptor<ReifiedJniTypeTrait,
+                                                        kMaxSize>;
+
+// Attempts to convert the function type T into a list of ReifiedJniTypeTraits
+// that correspond to the function components.
+//
+// If conversion fails (i.e. non-jni compatible types), then:
+//    parses are fatal -> assertion is triggered (default behavior),
+//    parses are nonfatal -> returns nullopt (test behavior).
+template <NativeKind native_kind,
+          typename T,
+          T fn,
+          size_t kMaxSize = FunctionTypeMetafunction<T, fn>::count>
+constexpr ConstexprOptional<ReifiedJniSignature<kMaxSize>>
+MaybeMakeReifiedJniSignature() {
+  if (!IsValidJniFunctionType<native_kind, T, fn>()) {
+    PARSE_FAILURE("The function signature has one or more types incompatible with JNI.");
+  }
+
+  ReifiedJniTypeTrait return_jni_trait =
+      FunctionTypeMetafunction<T,
+                         fn>::template map_return<ReifyJniTypeMetafunction>();
+
+  constexpr size_t
+      kSkipArgumentPrefix = (native_kind != kCriticalNative) ? 2u : 0u;
+  ConstexprVector<ReifiedJniTypeTrait, kMaxSize> args;
+  auto args_list =
+      FunctionTypeMetafunction<T, fn>::template map_args<ReifyJniTypeMetafunction>();
+  size_t args_index = 0;
+  for (auto& arg : args_list) {
+    // Ignore the 'JNIEnv*, jobject' / 'JNIEnv*, jclass' prefix,
+    // as its not part of the function descriptor string.
+    if (args_index >= kSkipArgumentPrefix) {
+      args.push_back(arg);
+    }
+
+    ++args_index;
+  }
+
+  return {{args, return_jni_trait}};
+}
+
+#define COMPARE_DESCRIPTOR_CHECK(expr) if (!(expr)) return false
+#define COMPARE_DESCRIPTOR_FAILURE_MSG(msg) if ((true)) return false
+
+// Compares a user-defined JNI descriptor (of a single argument or return value)
+// to a reified jni type trait that was derived from the C++ function type.
+//
+// If comparison fails (i.e. non-jni compatible types), then:
+//    parses are fatal -> assertion is triggered (default behavior),
+//    parses are nonfatal -> returns false (test behavior).
+constexpr bool
+CompareJniDescriptorNodeErased(JniDescriptorNode user_defined_descriptor,
+                               ReifiedJniTypeTrait derived) {
+
+  ConstexprOptional<ReifiedJniTypeTrait> user_reified_opt =
+      ReifiedJniTypeTrait::MostSimilarTypeDescriptor(user_defined_descriptor.longy);
+
+  if (!user_reified_opt.has_value()) {
+    COMPARE_DESCRIPTOR_FAILURE_MSG(
+        "Could not find any JNI C++ type corresponding to the type descriptor");
+  }
+
+  char user_shorty = user_defined_descriptor.longy.size() > 0 ?
+                     user_defined_descriptor.longy[0] :
+                     '\0';
+
+  ReifiedJniTypeTrait user = user_reified_opt.value();
+  if (user == derived) {
+    // If we had a similar match, immediately return success.
+    return true;
+  } else if (derived.type_name == "jthrowable") {
+    if (user_shorty == 'L') {
+      // Weakly allow any objects to correspond to a jthrowable.
+      // We do not know the managed type system so we have to be permissive here.
+      return true;
+    } else {
+      COMPARE_DESCRIPTOR_FAILURE_MSG(
+          "jthrowable must correspond to an object type descriptor");
+    }
+  } else if (derived.type_name == "jarray") {
+    if (user_shorty == '[') {
+      // a jarray is the base type for all other array types. Allow.
+      return true;
+    } else {
+      // Ljava/lang/Object; is the root for all array types.
+      // Already handled above in 'if user == derived'.
+      COMPARE_DESCRIPTOR_FAILURE_MSG(
+          "jarray must correspond to array type descriptor");
+    }
+  }
+  // Otherwise, the comparison has failed and the rest of this is only to
+  // pick the most appropriate error message.
+  //
+  // Note: A weaker form of comparison would allow matching 'Ljava/lang/String;'
+  // against 'jobject', etc. However the policy choice here is to enforce the strictest
+  // comparison that we can to utilize the type system to its fullest.
+
+  if (derived.type_finality == kFinal || user.type_finality == kFinal) {
+    // Final types, e.g. "I", "Ljava/lang/String;" etc must match exactly
+    // the C++ jni descriptor string ('I' -> jint, 'Ljava/lang/String;' -> jstring).
+    COMPARE_DESCRIPTOR_FAILURE_MSG(
+        "The JNI descriptor string must be the exact type equivalent of the "
+            "C++ function signature.");
+  } else if (user_shorty == '[') {
+    COMPARE_DESCRIPTOR_FAILURE_MSG(
+        "The array JNI descriptor must correspond to j${type}Array or jarray");
+  } else if (user_shorty == 'L') {
+    COMPARE_DESCRIPTOR_FAILURE_MSG(
+        "The object JNI descriptor must correspond to jobject.");
+  } else {
+    X_ASSERT(false);  // We should never get here, but either way this means the types did not match
+    COMPARE_DESCRIPTOR_FAILURE_MSG(
+        "The JNI type descriptor string does not correspond to the C++ JNI type.");
+  }
+}
+
+// Matches a user-defined JNI function descriptor against the C++ function type.
+//
+// If matches fails, then:
+//    parses are fatal -> assertion is triggered (default behavior),
+//    parses are nonfatal -> returns false (test behavior).
+template<NativeKind native_kind, typename T, T fn, size_t kMaxSize>
+constexpr bool
+MatchJniDescriptorWithFunctionType(ConstexprStringView user_function_descriptor) {
+  constexpr size_t kReifiedMaxSize = FunctionTypeMetafunction<T, fn>::count;
+
+  ConstexprOptional<ReifiedJniSignature<kReifiedMaxSize>>
+      reified_signature_opt =
+      MaybeMakeReifiedJniSignature<native_kind, T, fn>();
+  if (!reified_signature_opt) {
+    // Assertion handling done by MaybeMakeReifiedJniSignature.
+    return false;
+  }
+
+  ConstexprOptional<JniSignatureDescriptor<kMaxSize>> user_jni_sig_desc_opt =
+      ParseSignatureAsList<kMaxSize>(user_function_descriptor);
+
+  if (!user_jni_sig_desc_opt) {
+    // Assertion handling done by ParseSignatureAsList.
+    return false;
+  }
+
+  ReifiedJniSignature<kReifiedMaxSize>
+      reified_signature = reified_signature_opt.value();
+  JniSignatureDescriptor<kMaxSize>
+      user_jni_sig_desc = user_jni_sig_desc_opt.value();
+
+  if (reified_signature.args.size() != user_jni_sig_desc.args.size()) {
+    COMPARE_DESCRIPTOR_FAILURE_MSG(
+        "Number of parameters in JNI descriptor string"
+            "did not match number of parameters in C++ function type");
+  } else if (!CompareJniDescriptorNodeErased(user_jni_sig_desc.ret,
+                                             reified_signature.ret)) {
+    // Assertion handling done by CompareJniDescriptorNodeErased.
+    return false;
+  } else {
+    for (size_t i = 0; i < user_jni_sig_desc.args.size(); ++i) {
+      if (!CompareJniDescriptorNodeErased(user_jni_sig_desc.args[i],
+                                          reified_signature.args[i])) {
+        // Assertion handling done by CompareJniDescriptorNodeErased.
+        return false;
+      }
+    }
+  }
+
+  return true;
+}
+
+// Supports inferring the JNI function descriptor string from the C++
+// function type when all type components are final.
+template<NativeKind native_kind, typename T, T fn>
+struct InferJniDescriptor {
+  static constexpr size_t kMaxSize = FunctionTypeMetafunction<T, fn>::count;
+
+  // Convert the C++ function type into a JniSignatureDescriptor which holds
+  // the canonical (according to jni_traits) descriptors for each component.
+  // The C++ type -> JNI mapping must be nonambiguous (see jni_macros.h for exact rules).
+  //
+  // If conversion fails (i.e. C++ signatures is illegal for JNI, or the types are ambiguous):
+  //    if parsing is fatal -> assertion failure (default behavior)
+  //    if parsing is nonfatal -> returns nullopt (test behavior).
+  static constexpr ConstexprOptional<JniSignatureDescriptor<kMaxSize>> FromFunctionType() {
+    constexpr size_t kReifiedMaxSize = kMaxSize;
+    ConstexprOptional<ReifiedJniSignature<kReifiedMaxSize>>
+        reified_signature_opt =
+        MaybeMakeReifiedJniSignature<native_kind, T, fn>();
+    if (!reified_signature_opt) {
+      // Assertion handling done by MaybeMakeReifiedJniSignature.
+      return NullConstexprOptional{};
+    }
+
+    ReifiedJniSignature<kReifiedMaxSize>
+        reified_signature = reified_signature_opt.value();
+
+    JniSignatureDescriptor<kReifiedMaxSize> signature_descriptor;
+
+    if (reified_signature.ret.type_finality != kFinal) {
+      // e.g. jint, jfloatArray, jstring, jclass are ok. jobject, jthrowable, jarray are not.
+      PARSE_FAILURE("Bad return type. Only unambigous (final) types can be used to infer a signature.");  // NOLINT
+    }
+    signature_descriptor.ret =
+        JniDescriptorNode{reified_signature.ret.type_descriptor};
+
+    for (size_t i = 0; i < reified_signature.args.size(); ++i) {
+      const ReifiedJniTypeTrait& arg_trait = reified_signature.args[i];
+      if (arg_trait.type_finality != kFinal) {
+        PARSE_FAILURE("Bad parameter type. Only unambigous (final) types can be used to infer a signature.");  // NOLINT
+      }
+      signature_descriptor.args.push_back(JniDescriptorNode{
+          arg_trait.type_descriptor});
+    }
+
+    return {signature_descriptor};
+  }
+
+  // Calculate the exact string size that the JNI descriptor will be
+  // at runtime.
+  //
+  // Without this we cannot allocate enough space within static storage
+  // to fit the compile-time evaluated string.
+  static constexpr size_t CalculateStringSize() {
+    ConstexprOptional<JniSignatureDescriptor<kMaxSize>>
+        signature_descriptor_opt =
+        FromFunctionType();
+    if (!signature_descriptor_opt) {
+      // Assertion handling done by FromFunctionType.
+      return 0u;
+    }
+
+    JniSignatureDescriptor<kMaxSize> signature_descriptor =
+        signature_descriptor_opt.value();
+
+    size_t acc_size = 1u;  // All sigs start with '('.
+
+    // Now add every parameter.
+    for (size_t j = 0; j < signature_descriptor.args.size(); ++j) {
+      const JniDescriptorNode& arg_descriptor = signature_descriptor.args[j];
+      // for (const JniDescriptorNode& arg_descriptor : signature_descriptor.args) {
+      acc_size += arg_descriptor.longy.size();
+    }
+
+    acc_size += 1u;   // Add space for ')'.
+
+    // Add space for the return value.
+    acc_size += signature_descriptor.ret.longy.size();
+
+    return acc_size;
+  }
+
+  static constexpr size_t kMaxStringSize = CalculateStringSize();
+  using ConstexprStringDescriptorType = ConstexprArray<char,
+                                                       kMaxStringSize + 1>;
+
+  static constexpr bool kAllowPartialStrings = false;
+
+  // Convert the JniSignatureDescriptor we get in FromFunctionType()
+  // into a flat constexpr char array.
+  //
+  // This is done by repeated string concatenation at compile-time.
+  static constexpr ConstexprStringDescriptorType GetString() {
+    ConstexprStringDescriptorType c_str{};
+
+    ConstexprOptional<JniSignatureDescriptor<kMaxSize>>
+        signature_descriptor_opt =
+        FromFunctionType();
+    if (!signature_descriptor_opt.has_value()) {
+      // Assertion handling done by FromFunctionType.
+      c_str[0] = '\0';
+      return c_str;
+    }
+
+    JniSignatureDescriptor<kMaxSize> signature_descriptor =
+        signature_descriptor_opt.value();
+
+    size_t pos = 0u;
+    c_str[pos++] = '(';
+
+    // Copy all parameter descriptors.
+    for (size_t j = 0; j < signature_descriptor.args.size(); ++j) {
+      const JniDescriptorNode& arg_descriptor = signature_descriptor.args[j];
+      ConstexprStringView longy = arg_descriptor.longy;
+      for (size_t i = 0; i < longy.size(); ++i) {
+        if (kAllowPartialStrings && pos >= kMaxStringSize) {
+          break;
+        }
+        c_str[pos++] = longy[i];
+      }
+    }
+
+    if (!kAllowPartialStrings || pos < kMaxStringSize) {
+      c_str[pos++] = ')';
+    }
+
+    // Copy return descriptor.
+    ConstexprStringView longy = signature_descriptor.ret.longy;
+    for (size_t i = 0; i < longy.size(); ++i) {
+      if (kAllowPartialStrings && pos >= kMaxStringSize) {
+        break;
+      }
+      c_str[pos++] = longy[i];
+    }
+
+    if (!kAllowPartialStrings) {
+      X_ASSERT(pos == kMaxStringSize);
+    }
+
+    c_str[pos] = '\0';
+
+    return c_str;
+  }
+
+  // Turn a pure constexpr string into one that can be accessed at non-constexpr
+  // time. Note that the 'static constexpr' storage must be in the scope of a
+  // function (prior to C++17) to avoid linking errors.
+  static const char* GetStringAtRuntime() {
+    static constexpr ConstexprStringDescriptorType str = GetString();
+    return &str[0];
+  }
+};
+
+// Expression to return JNINativeMethod, performs checking on signature+fn.
+#define MAKE_CHECKED_JNI_NATIVE_METHOD(native_kind, name_, signature_, fn) \
+  ([]() {                                                                \
+    using namespace nativehelper::detail;                                \
+    static_assert(                                                       \
+        MatchJniDescriptorWithFunctionType<native_kind,                  \
+                                           decltype(fn),                 \
+                                           fn,                           \
+                                           sizeof(signature_)>(signature_),\
+        "JNI signature doesn't match C++ function type.");               \
+    /* Suppress implicit cast warnings by explicitly casting. */         \
+    return JNINativeMethod {                                             \
+        const_cast<decltype(JNINativeMethod::name)>(name_),              \
+        const_cast<decltype(JNINativeMethod::signature)>(signature_),    \
+        reinterpret_cast<void*>(&fn)};                                   \
+  })()
+
+// Expression to return JNINativeMethod, infers signature from fn.
+#define MAKE_INFERRED_JNI_NATIVE_METHOD(native_kind, name_, fn)          \
+  ([]() {                                                                \
+    using namespace nativehelper::detail;                                \
+    /* Suppress implicit cast warnings by explicitly casting. */         \
+    return JNINativeMethod {                                             \
+        const_cast<decltype(JNINativeMethod::name)>(name_),              \
+        const_cast<decltype(JNINativeMethod::signature)>(                \
+            InferJniDescriptor<native_kind,                              \
+                               decltype(fn),                             \
+                               fn>::GetStringAtRuntime()),               \
+        reinterpret_cast<void*>(&fn)};                                   \
+  })()
+
+}  // namespace detail
+}  // namespace nativehelper
+
diff --git a/platform_include/nativehelper/jni_macros.h b/platform_include/nativehelper/jni_macros.h
index b628bf7..1276038 100644
--- a/platform_include/nativehelper/jni_macros.h
+++ b/platform_include/nativehelper/jni_macros.h
@@ -14,71 +14,276 @@
  * limitations under the License.
  */
 
-/*
- * JNI helper macros.
+/**
+ * Compile-time, zero-cost checking of JNI signatures against their C++ function type.
+ * This can trigger compile-time assertions if any of the input is invalid:
+ *     (a) The signature specified does not conform to the JNI function descriptor syntax.
+ *     (b) The C++ function is itself an invalid JNI function (e.g. missing JNIEnv*, etc).
+ *     (c) The descriptor does not match the C++ function (e.g. "()V" will not match jint(jint)).
  *
- * Only intended to be used in the platform.
+ * The fundamental macros are as following:
+ *   MAKE_JNI_[FAST_|CRITICAL_]NATIVE_METHOD - Create a checked JNINativeMethod{name, sig, func}.
+ *   MAKE_JNI_[FAST_|CRITICAL_]NATIVE_METHOD_AUTOSIG - Same as above, but infer the JNI signature.
+ *
+ * Usage examples:
+ *     // path/to/package/KlassName.java
+ *     class KlassName {
+ *         native jobject normal(int x);
+ *         @FastNative native jobject fast(int x);
+ *         @CriticalNative native int critical(long ptr);
+ *     }
+ *     // path_to_package_KlassName.cpp
+ *     jobject KlassName_normal(JNIEnv*,jobject,jint) {...}
+ *     jobject KlassName_fast(JNIEnv*,jobject,jint) {...}
+ *     jint KlassName_critical(jlong) {...}
+ *
+ *     // Manually specify each signature:
+ *     JNINativeMethod[] gMethods = {
+ *         MAKE_JNI_NATIVE_METHOD("normal", "(I)Ljava/lang/Object;", KlassName_normal),
+ *         MAKE_JNI_FAST_NATIVE_METHOD("fast", "(I)Ljava/lang/Object;", KlassName_fast),
+ *         MAKE_JNI_CRITICAL_NATIVE_METHOD("critical", "(Z)I", KlassName_critical),
+ *     };
+ *
+ *     // Automatically infer the signature:
+ *     JNINativeMethod[] gMethodsAutomaticSignature = {
+ *         MAKE_JNI_NATIVE_METHOD_AUTOSIG("normal", KlassName_normal),
+ *         MAKE_JNI_FAST_NATIVE_METHOD_AUTOSIG("fast", KlassName_fast),
+ *         MAKE_JNI_CRITICAL_NATIVE_METHOD_AUTOSIG("critical", KlassName_critical),
+ *     };
+ *
+ *     // and then call JNIEnv::RegisterNatives with gMethods as usual.
+ *
+ * For convenience the following macros are defined:
+ *   [FAST_|CRITICAL_]NATIVE_METHOD - Return JNINativeMethod for class, func name, and signature.
+ *   OVERLOADED_[FAST_|CRITICAL_]NATIVE_METHOD - Same as above but allows a separate func identifier.
+ *   [FAST_|CRITICAL_]NATIVE_METHOD_AUTOSIG - Return JNINativeMethod, sig inferred from function.
+ *
+ * The FAST_ prefix corresponds to functions annotated with @FastNative,
+ * and the CRITICAL_ prefix corresponds to functions annotated with @CriticalNative.
+ * See dalvik.annotation.optimization.CriticalNative for more details.
+ *
+ * =======================================
+ * Checking rules
+ * =======================================
+ *
+ * ---------------------------------------
+ * JNI descriptor syntax for functions
+ *
+ * Refer to "Chapter 3: JNI Types and Data Structures" of the JNI specification
+ * under the subsection "Type Signatures" table entry "method type".
+ *
+ * JNI signatures not conforming to the above syntax are rejected.
+ * ---------------------------------------
+ * C++ function types
+ *
+ * A normal or @FastNative JNI function type must be of the form
+ *
+ *     ReturnType (JNIEnv*, jclass|jobject, [ArgTypes...]) {}
+ *
+ * A @CriticalNative JNI function type:
+ *
+ *   must be of the form...  ReturnType ([ArgTypes...]){}
+ *   and must not contain any Reference Types.
+ *
+ * Refer to "Chapter 3: JNI Types and Data Structures" of the JNI specification
+ * under the subsection "Primitive Types" and "Reference Types" for the list
+ * of valid argument/return types.
+ *
+ * C++ function types not conforming to the above requirements are rejected.
+ * ---------------------------------------
+ * Matching of C++ function type against JNI function descriptor.
+ *
+ * Assuming all of the above conditions are met for signature and C++ type validity,
+ * then matching between the signature and the type validity can occur:
+ *
+ * Given a signature (Args...)Ret and the
+ *     C++ function type of the form "CRet fn(JNIEnv*, jclass|jobject, CArgs...)",
+ *     or for @CriticalNative of the form "CRet fn(CArgs...)"
+ *
+ * The number of Args... and the number of CArgs... must be equal.
+ *
+ * If so, attempt to match every component from the signature and function type
+ * against each other:
+ *
+ * ReturnType:
+ *     V <-> void
+ *     ArgumentType
+ *
+ * ArgumentType:
+ *     PrimitiveType
+ *     ReferenceType  [except for @CriticalNative]
+ *
+ * PrimitiveType:
+ *     Z <-> jboolean
+ *     B <-> jbyte
+ *     C <-> jchar
+ *     S <-> jshort
+ *     I <-> jint
+ *     J <-> jlong
+ *     F <-> jfloat
+ *     D <-> jdouble
+ *
+ * ReferenceType:
+ *     Ljava/lang/String;    <-> jstring
+ *     Ljava/lang/Class;     <-> jclass
+ *     L*;                   <-  jobject
+ *     Ljava/lang/Throwable;  -> jthrowable
+ *     L*;                   <-  jthrowable
+ *     [ PrimitiveType       <-> ${CPrimitiveType}Array
+ *     [ ReferenceType       <-> jobjectArray
+ *     [*                    <-  jarray
+ *
+ * Wherein <-> represents a strong match (if the left or right pattern occurs,
+ * then left must match right, otherwise matching fails). <- and -> represent
+ * weak matches (that is, other match rules can be still attempted).
+ *
+ * Sidenote: Whilst a jobject could also represent a jclass, jstring, etc,
+ * the stricter approach is taken: the most exact C++ type must be used.
  */
 
 #ifndef NATIVEHELPER_JNI_MACROS_H
 #define NATIVEHELPER_JNI_MACROS_H
 
+// The below basic macros do not perform automatic stringification,
+// invoked e.g. as MAKE_JNI_NATIVE_METHOD("some_name", "()V", void_fn)
 
+// An expression that evaluates to JNINativeMethod { name, signature, function },
+//   and applies the above compile-time checking for signature+function.
+// The equivalent Java Language code must not be annotated with @FastNative/@CriticalNative.
+#define MAKE_JNI_NATIVE_METHOD(name, signature, function)                      \
+  _NATIVEHELPER_JNI_MAKE_METHOD(kNormalNative, name, signature, function)
+
+// An expression that evaluates to JNINativeMethod { name, signature, function },
+//   and applies the above compile-time checking for signature+function.
+// The equivalent Java Language code must be annotated with @FastNative.
+#define MAKE_JNI_FAST_NATIVE_METHOD(name, signature, function)                 \
+  _NATIVEHELPER_JNI_MAKE_METHOD(kFastNative, name, signature, function)
+
+// An expression that evaluates to JNINativeMethod { name, signature, function },
+//   and applies the above compile-time checking for signature+function.
+// The equivalent Java Language code must be annotated with @CriticalNative.
+#define MAKE_JNI_CRITICAL_NATIVE_METHOD(name, signature, function)             \
+  _NATIVEHELPER_JNI_MAKE_METHOD(kCriticalNative, name, signature, function)
+
+// Automatically signature-inferencing macros are also available,
+// which also checks the C++ function types for validity:
+
+// An expression that evalutes to JNINativeMethod { name, infersig(function), function) }
+// by inferring the signature at compile-time. Only works when the C++ function type
+// corresponds to one unambigous JNI parameter (e.g. 'jintArray' -> '[I' but 'jobject' -> ???).
+//
+// The equivalent Java Language code must not be annotated with @FastNative/@CriticalNative.
+#define MAKE_JNI_NATIVE_METHOD_AUTOSIG(name, function)                         \
+  _NATIVEHELPER_JNI_MAKE_METHOD_AUTOSIG(kNormalNative, name, function)
+
+// An expression that evalutes to JNINativeMethod { name, infersig(function), function) }
+// by inferring the signature at compile-time. Only works when the C++ function type
+// corresponds to one unambigous JNI parameter (e.g. 'jintArray' -> '[I' but 'jobject' -> ???).
+//
+// The equivalent Java Language code must be annotated with @FastNative.
+#define MAKE_JNI_FAST_NATIVE_METHOD_AUTOSIG(name, function)                    \
+  _NATIVEHELPER_JNI_MAKE_METHOD_AUTOSIG(kFastNative, name, function)
+
+// An expression that evalutes to JNINativeMethod { name, infersig(function), function) }
+// by inferring the signature at compile-time.
+//
+// The equivalent Java Language code must be annotated with @CriticalNative.
+#define MAKE_JNI_CRITICAL_NATIVE_METHOD_AUTOSIG(name, function)                 \
+  _NATIVEHELPER_JNI_MAKE_METHOD_AUTOSIG(kCriticalNative, name, function)
+
+// Convenience macros when the functions follow the naming convention:
+//       .java file           .cpp file
+//       JavaLanguageName <-> ${ClassName}_${JavaLanguageName}
+//
+// Stringification is done automatically, invoked as:
+//   NATIVE_[FAST_|CRITICAL]_METHOD(ClassName, JavaLanguageName, Signature)
+//
 // Intended to construct a JNINativeMethod.
 //   (Assumes the C name is the ClassName_JavaMethodName).
-#ifndef NATIVE_METHOD
-#define NATIVE_METHOD(className, functionName, signature)                \
-  { #functionName,                                                       \
-    signature,                                                           \
-    _NATIVEHELPER_JNI_MACRO_CAST(void*) (className ## _ ## functionName) \
-  }
-#endif
-
-// Intended to construct a JNINativeMethod (when the C name doesn't match the Java name).
-//   (Assumes the C name is the ClassName_Identifier).
-#ifndef OVERLOADED_NATIVE_METHOD
-#define OVERLOADED_NATIVE_METHOD(className, functionName, signature, identifier) \
-  { #functionName,                                                               \
-    signature,                                                                   \
-    _NATIVEHELPER_JNI_MACRO_CAST(void*) (className ## _ ## identifier)           \
-  }
-#endif
-
-// Used for methods that are annotated with @FastNative on the managed side.
-//   See NATIVE_METHOD for usage.
-#ifndef FAST_NATIVE_METHOD
-#define FAST_NATIVE_METHOD(className, functionName, signature)           \
-  { #functionName,                                                       \
-    signature,                                                           \
-    _NATIVEHELPER_JNI_MACRO_CAST(void*) (className ## _ ## functionName) \
-  }
-#endif
-
-// Used for methods that are annotated with @FastNative on the managed side,
-//   and when the C-name doesn't match the Java-name.
 //
-//   See OVERLOADED_NATIVE_METHOD for usage.
-#ifndef OVERLOADED_FAST_NATIVE_METHOD
-#define OVERLOADED_FAST_NATIVE_METHOD(className, functionName, signature, identifier) \
-  { #functionName,                                                                    \
-    signature,                                                                        \
-    _NATIVEHELPER_JNI_MACRO_CAST(void*) (className ## _ ## identifier)                \
-  }
+// The Java Language code must be annotated with one of (none,@FastNative,@CriticalNative)
+// for the (none,FAST_,CRITICAL_) variants of these macros.
+
+#ifdef NATIVE_METHOD  // Remove definition from JniConstants.h
+#undef NATIVE_METHOD
 #endif
 
+#define NATIVE_METHOD(className, functionName, signature)                \
+  MAKE_JNI_NATIVE_METHOD(#functionName, signature, className ## _ ## functionName)
+
+#define OVERLOADED_NATIVE_METHOD(className, functionName, signature, identifier) \
+  MAKE_JNI_NATIVE_METHOD(#functionName, signature, className ## _ ## identifier)
+
+#define NATIVE_METHOD_AUTOSIG(className, functionName) \
+  MAKE_JNI_NATIVE_METHOD_AUTOSIG(#functionName, className ## _ ## functionName)
+
+#define FAST_NATIVE_METHOD(className, functionName, signature)           \
+  MAKE_JNI_FAST_NATIVE_METHOD(#functionName, signature, className ## _ ## functionName)
+
+#define OVERLOADED_FAST_NATIVE_METHOD(className, functionName, signature, identifier) \
+  MAKE_JNI_FAST_NATIVE_METHOD(#functionName, signature, className ## _ ## identifier)
+
+#define FAST_NATIVE_METHOD_AUTOSIG(className, functionName) \
+  MAKE_JNI_FAST_NATIVE_METHOD_AUTOSIG(#functionName, className ## _ ## functionName)
+
+#define CRITICAL_NATIVE_METHOD(className, functionName, signature)           \
+  MAKE_JNI_CRITICAL_NATIVE_METHOD(#functionName, signature, className ## _ ## functionName)
+
+#define OVERLOADED_CRITICAL_NATIVE_METHOD(className, functionName, signature, identifier) \
+  MAKE_JNI_CRITICAL_NATIVE_METHOD(#functionName, signature, className ## _ ## identifier)
+
+#define CRITICAL_NATIVE_METHOD_AUTOSIG(className, functionName) \
+  MAKE_JNI_CRITICAL_NATIVE_METHOD_AUTOSIG(#functionName, className ## _ ## functionName)
+
 ////////////////////////////////////////////////////////
 //                IMPLEMENTATION ONLY.
 //                DO NOT USE DIRECTLY.
 ////////////////////////////////////////////////////////
 
+#if defined(__cplusplus) && __cplusplus >= 201402L
+#include "nativehelper/detail/signature_checker.h"  // for MAKE_CHECKED_JNI_NATIVE_METHOD
+#endif
+
+// Expands to an expression whose type is JNINativeMethod.
+// This is for older versions of C++ or C, so it has no compile-time checking.
+#define _NATIVEHELPER_JNI_MAKE_METHOD_OLD(kind, name, sig, fn)     \
+  (                                                                \
+    (JNINativeMethod) {                                            \
+        (name),                                                    \
+        (sig),                                                     \
+        _NATIVEHELPER_JNI_MACRO_CAST(reinterpret_cast, void *)(fn) \
+    }                                                             \
+  )
+
+// C++14 or better, use compile-time checking.
+#if defined(__cplusplus) && __cplusplus >= 201402L
+// Expands to a compound expression whose type is JNINativeMethod.
+#define _NATIVEHELPER_JNI_MAKE_METHOD(kind, name, sig, fn) \
+  MAKE_CHECKED_JNI_NATIVE_METHOD(kind, name, sig, fn)
+
+// Expands to a compound expression whose type is JNINativeMethod.
+#define _NATIVEHELPER_JNI_MAKE_METHOD_AUTOSIG(kind, name, function) \
+  MAKE_INFERRED_JNI_NATIVE_METHOD(kind, name, function)
+
+#else
+// Older versions of C++ or C code get the regular macro that's unchecked.
+// Expands to a compound expression whose type is JNINativeMethod.
+#define _NATIVEHELPER_JNI_MAKE_METHOD(kind, name, sig, fn)         \
+  _NATIVEHELPER_JNI_MAKE_METHOD_OLD(kind, name, sig, fn)
+
+// Need C++14 or newer to use the AUTOSIG macros.
+#define _NATIVEHELPER_JNI_MAKE_METHOD_AUTOSIG(kind, name, function) \
+  static_assert(false, "Cannot infer JNI signatures prior to C++14 for function " #function);
+
+#endif  // C++14 check
 
 // C-style cast for C, C++-style cast for C++ to avoid warnings/errors.
 #if defined(__cplusplus)
-#define _NATIVEHELPER_JNI_MACRO_CAST(to) \
-    reinterpret_cast<to>
+#define _NATIVEHELPER_JNI_MACRO_CAST(which_cast, to) \
+    which_cast<to>
 #else
-#define _NATIVEHELPER_JNI_MACRO_CAST(to) \
+#define _NATIVEHELPER_JNI_MACRO_CAST(which_cast, to) \
     (to)
 #endif
 
-#endif
+#endif  // NATIVEHELPER_JNI_MACROS_H
diff --git a/tests/Android.bp b/tests/Android.bp
index e6cbf5c..a5bdee9 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -8,3 +8,53 @@
     cflags: ["-Wall", "-Werror"],
     shared_libs: ["libnativehelper"],
 }
+
+cc_test {
+  name: "JniSafeRegisterNativeMethods_test",
+  host_supported: true,
+  srcs: ["JniSafeRegisterNativeMethods_test.cpp"],
+
+  cflags: [
+      // Base set of cflags used by all things ART.
+      "-fno-rtti",
+      "-ggdb3",
+      "-Wall",
+      "-Werror",
+      "-Wextra",
+      "-Wstrict-aliasing",
+      "-fstrict-aliasing",
+      "-Wunreachable-code",
+      "-Wredundant-decls",
+      "-Wshadow",
+      "-Wunused",
+      "-fvisibility=protected",
+
+      // Warn about thread safety violations with clang.
+      "-Wthread-safety",
+      "-Wthread-safety-negative",
+
+      // Warn if switch fallthroughs aren't annotated.
+      "-Wimplicit-fallthrough",
+
+      // Enable float equality warnings.
+      "-Wfloat-equal",
+
+      // Enable warning of converting ints to void*.
+      "-Wint-to-void-pointer-cast",
+
+      // Enable warning of wrong unused annotations.
+      "-Wused-but-marked-unused",
+
+      // Enable warning for deprecated language features.
+      "-Wdeprecated",
+
+      // Enable warning for unreachable break & return.
+      "-Wunreachable-code-break",
+      "-Wunreachable-code-return",
+
+      // Enable thread annotations for std::mutex, etc.
+      "-D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS",
+  ],
+
+  shared_libs: ["libnativehelper"],
+}
diff --git a/tests/JniSafeRegisterNativeMethods_test.cpp b/tests/JniSafeRegisterNativeMethods_test.cpp
new file mode 100644
index 0000000..5a1f8e9
--- /dev/null
+++ b/tests/JniSafeRegisterNativeMethods_test.cpp
@@ -0,0 +1,882 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wused-but-marked-unused"
+#pragma clang diagnostic ignored "-Wdeprecated-dynamic-exception-spec"
+#pragma clang diagnostic ignored "-Wdeprecated"
+#include <gtest/gtest.h>
+#pragma clang diagnostic pop
+#include <sstream>
+
+#define PARSE_FAILURES_NONFATAL  // return empty optionals wherever possible instead of asserting.
+#include "nativehelper/jni_macros.h"
+
+// Provide static storage to these values so they can be used in a runtime context.
+// This has to be defined local to the test translation unit to avoid ODR violations prior to C++17.
+#define STORAGE_FN_FOR_JNI_TRAITS(jtype, ...)                                  \
+constexpr char nativehelper::detail::jni_type_trait<jtype>::type_descriptor[]; \
+constexpr char nativehelper::detail::jni_type_trait<jtype>::type_name[];
+
+DEFINE_JNI_TYPE_TRAIT(STORAGE_FN_FOR_JNI_TRAITS)
+
+template <typename T>
+auto stringify_helper(const T& val) -> decltype(std::stringstream().str()) {  // suppress incorrect warnings about compiler not support 'auto'
+  std::stringstream ss;
+  ss << val;
+  return ss.str();
+}
+
+#define EXPECT_STRINGIFY_EQ(x, y) EXPECT_EQ(stringify_helper(x), stringify_helper(y))
+
+TEST(JniSafeRegisterNativeMethods, StringParsing) {
+  using namespace nativehelper::detail;                                \
+
+  // Super basic bring-up tests for core functionality.
+
+  {
+    constexpr ConstexprStringView v_str = "V";
+    EXPECT_EQ(1u, v_str.size());
+    EXPECT_EQ(false, v_str.empty());
+
+    std::stringstream ss;
+    ss << v_str;
+    EXPECT_EQ("V", ss.str());
+  }
+
+  {
+    auto parse = ParseSingleTypeDescriptor("", /*allow_void*/true);
+    EXPECT_EQ("", parse->token);
+    EXPECT_EQ("", parse->remainder);
+  }
+
+  {
+    auto parse = ParseSingleTypeDescriptor("V", /*allow_void*/true);
+    EXPECT_EQ("V", parse->token);
+    EXPECT_EQ("", parse->remainder);
+  }
+
+  {
+    auto parse = ParseSingleTypeDescriptor("[I");
+    EXPECT_EQ("[I", parse->token);
+    EXPECT_EQ("", parse->remainder);
+  }
+
+  // Stringify is used for convenience to make writing out tests easier.
+  // Transforms as "(XYZ)W" -> "args={X,Y,Z}, ret=W"
+
+#define PARSE_SIGNATURE_AS_LIST(str) (ParseSignatureAsList<sizeof(str)>(str))
+
+  {
+    constexpr auto jni_descriptor = PARSE_SIGNATURE_AS_LIST("()V");
+    EXPECT_STRINGIFY_EQ("args={}, ret=V", jni_descriptor);
+  }
+
+  {
+    constexpr auto
+        jni_descriptor = PARSE_SIGNATURE_AS_LIST("()Ljava/lang/Object;");
+    EXPECT_STRINGIFY_EQ("args={}, ret=Ljava/lang/Object;", jni_descriptor);
+  }
+
+  {
+    constexpr auto jni_descriptor = PARSE_SIGNATURE_AS_LIST("()[I");
+    EXPECT_STRINGIFY_EQ("args={}, ret=[I", jni_descriptor);
+  }
+
+#define EXPECT_OK_SIGNATURE_PARSE(signature, args, ret) \
+  do { \
+    constexpr auto jni_descriptor = PARSE_SIGNATURE_AS_LIST(signature); \
+    EXPECT_EQ(true, jni_descriptor.has_value());                        \
+    EXPECT_STRINGIFY_EQ("args={" args "}, ret=" ret, jni_descriptor);   \
+  } while (0)
+
+  // Exhaustive tests for successful parsing.
+
+  EXPECT_OK_SIGNATURE_PARSE("()V", /*args*/"", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("()Z", /*args*/"", /*ret*/"Z");
+  EXPECT_OK_SIGNATURE_PARSE("()B", /*args*/"", /*ret*/"B");
+  EXPECT_OK_SIGNATURE_PARSE("()C", /*args*/"", /*ret*/"C");
+  EXPECT_OK_SIGNATURE_PARSE("()S", /*args*/"", /*ret*/"S");
+  EXPECT_OK_SIGNATURE_PARSE("()I", /*args*/"", /*ret*/"I");
+  EXPECT_OK_SIGNATURE_PARSE("()F", /*args*/"", /*ret*/"F");
+  EXPECT_OK_SIGNATURE_PARSE("()J", /*args*/"", /*ret*/"J");
+  EXPECT_OK_SIGNATURE_PARSE("()D", /*args*/"", /*ret*/"D");
+  EXPECT_OK_SIGNATURE_PARSE("()Ljava/lang/Object;", /*args*/
+                            "", /*ret*/
+                            "Ljava/lang/Object;");
+  EXPECT_OK_SIGNATURE_PARSE("()[Ljava/lang/Object;", /*args*/
+                            "", /*ret*/
+                            "[Ljava/lang/Object;");
+  EXPECT_OK_SIGNATURE_PARSE("()[I", /*args*/"", /*ret*/"[I");
+  EXPECT_OK_SIGNATURE_PARSE("()[[I", /*args*/"", /*ret*/"[[I");
+  EXPECT_OK_SIGNATURE_PARSE("()[[[I", /*args*/"", /*ret*/"[[[I");
+
+
+  EXPECT_OK_SIGNATURE_PARSE("(Z)V", /*args*/"Z", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(B)V", /*args*/"B", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(C)D", /*args*/"C", /*ret*/"D");
+  EXPECT_OK_SIGNATURE_PARSE("(S)V", /*args*/"S", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(I)V", /*args*/"I", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(F)V", /*args*/"F", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(J)F", /*args*/"J", /*ret*/"F");
+  EXPECT_OK_SIGNATURE_PARSE("(D)V", /*args*/"D", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(Ljava/lang/Object;)V", "Ljava/lang/Object;", "V");
+  EXPECT_OK_SIGNATURE_PARSE("([Ljava/lang/Object;)V",
+                            "[Ljava/lang/Object;",
+                            "V");
+  EXPECT_OK_SIGNATURE_PARSE("([I)V", /*ret*/"[I", "V");
+  EXPECT_OK_SIGNATURE_PARSE("([[I)V", /*ret*/"[[I", "V");
+  EXPECT_OK_SIGNATURE_PARSE("([[[I)V", /*ret*/"[[[I", "V");
+
+  EXPECT_OK_SIGNATURE_PARSE("(ZIJ)V", /*args*/"Z,I,J", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(B[IJ)V", /*args*/"B,[I,J", /*ret*/"V");
+  EXPECT_OK_SIGNATURE_PARSE("(Ljava/lang/Object;B)D", /*args*/
+                            "Ljava/lang/Object;,B", /*ret*/
+                            "D");
+  EXPECT_OK_SIGNATURE_PARSE("(Ljava/lang/Object;Ljava/lang/String;IF)D", /*args*/
+                            "Ljava/lang/Object;,Ljava/lang/String;,I,F", /*ret*/
+                            "D");
+  EXPECT_OK_SIGNATURE_PARSE("([[[Ljava/lang/Object;Ljava/lang/String;IF)D", /*args*/
+                            "[[[Ljava/lang/Object;,Ljava/lang/String;,I,F", /*ret*/
+                            "D");
+
+  /*
+   * Test Failures in Parsing
+   */
+
+#define EXPECT_FAILED_SIGNATURE_PARSE(jni_descriptor) \
+  EXPECT_STRINGIFY_EQ(ConstexprOptional<JniSignatureDescriptor<sizeof(jni_descriptor)>>{},\
+                      ParseSignatureAsList<sizeof(jni_descriptor)>(jni_descriptor))
+
+  // For the failures to work we must turn off 'PARSE_FAILURES_FATAL'.
+  // Otherwise they immediately cause a crash, which is actually the desired behavior
+  // when this is used by the end-user in REGISTER_NATIVE_METHOD.
+  {
+    EXPECT_FAILED_SIGNATURE_PARSE("");
+    EXPECT_FAILED_SIGNATURE_PARSE("A");
+    EXPECT_FAILED_SIGNATURE_PARSE(")");
+    EXPECT_FAILED_SIGNATURE_PARSE("V");
+    EXPECT_FAILED_SIGNATURE_PARSE("(");
+    EXPECT_FAILED_SIGNATURE_PARSE("(A");
+    EXPECT_FAILED_SIGNATURE_PARSE("()");
+    EXPECT_FAILED_SIGNATURE_PARSE("()A");
+    EXPECT_FAILED_SIGNATURE_PARSE("()VV");
+    EXPECT_FAILED_SIGNATURE_PARSE("()L");
+    EXPECT_FAILED_SIGNATURE_PARSE("()L;");
+    EXPECT_FAILED_SIGNATURE_PARSE("()BAD;");
+    EXPECT_FAILED_SIGNATURE_PARSE("()Ljava/lang/Object");
+    EXPECT_FAILED_SIGNATURE_PARSE("()Ljava/lang/Object;X");
+
+    EXPECT_FAILED_SIGNATURE_PARSE("(V)V");
+    EXPECT_FAILED_SIGNATURE_PARSE("(ILcat)V");
+    EXPECT_FAILED_SIGNATURE_PARSE("([dog)V");
+    EXPECT_FAILED_SIGNATURE_PARSE("(IV)V");
+    EXPECT_FAILED_SIGNATURE_PARSE("([V)V");
+    EXPECT_FAILED_SIGNATURE_PARSE("([[V)V");
+    EXPECT_FAILED_SIGNATURE_PARSE("()v");
+    EXPECT_FAILED_SIGNATURE_PARSE("()i");
+    EXPECT_FAILED_SIGNATURE_PARSE("()f");
+  }
+
+}
+
+#define EXPECT_IS_VALID_JNI_ARGUMENT_TYPE(expected, expr) \
+ { constexpr bool is_valid = (expr); \
+   EXPECT_EQ(expected, is_valid) << #expr; \
+ }
+
+// Basic smoke tests for parameter validity.
+// See below for more exhaustive tests.
+TEST(JniSafeRegisterNativeMethods, ParameterTypes) {
+  using namespace nativehelper::detail;
+  EXPECT_TRUE(IsJniParameterCountValid(kCriticalNative, 0u));
+  EXPECT_TRUE(IsJniParameterCountValid(kCriticalNative, 1u));
+  EXPECT_TRUE(IsJniParameterCountValid(kCriticalNative, 2u));
+  EXPECT_TRUE(IsJniParameterCountValid(kCriticalNative, 3u));
+  EXPECT_TRUE(IsJniParameterCountValid(kCriticalNative, 4u));
+
+  EXPECT_FALSE(IsJniParameterCountValid(kNormalNative, 0u));
+  EXPECT_FALSE(IsJniParameterCountValid(kNormalNative, 1u));
+  EXPECT_TRUE(IsJniParameterCountValid(kNormalNative, 2u));
+  EXPECT_TRUE(IsJniParameterCountValid(kNormalNative, 3u));
+  EXPECT_TRUE(IsJniParameterCountValid(kNormalNative, 4u));
+
+  EXPECT_TRUE((IsValidJniParameter<void>(kNormalNative, kReturnPosition)));
+  EXPECT_IS_VALID_JNI_ARGUMENT_TYPE(true,(is_valid_jni_argument_type<kNormalNative, /*pos*/0u, JNIEnv*>::value));
+  EXPECT_IS_VALID_JNI_ARGUMENT_TYPE(true,(is_valid_jni_argument_type<kNormalNative, /*pos*/1u, jobject>::value));
+  EXPECT_IS_VALID_JNI_ARGUMENT_TYPE(true,(is_valid_jni_argument_type<kNormalNative, /*pos*/1u, jclass>::value));
+  EXPECT_IS_VALID_JNI_ARGUMENT_TYPE(false,(is_valid_jni_argument_type<kNormalNative, /*pos*/1u, jstring>::value));
+}
+
+struct TestReturnAnything {
+  template <typename T>
+  operator T() const {
+    return T{};
+  }
+};
+
+namespace test_jni {
+  void empty_fn() {}
+}
+struct TestJni {
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-parameter"
+
+  // Always bad.
+  static void bad_cptr(const char* ptr) {}
+  static void* bad_ret_ptr() { return nullptr; }
+  static JNIEnv* bad_ret_env() { return nullptr; }
+  static void bad_wrongplace_env(jobject, JNIEnv*) {}
+  static void bad_wrongplace_env2(jobject, jobject, JNIEnv*) {}
+  static void v_e(JNIEnv*) {}
+  static void v_ei(JNIEnv*, jint l) {}
+  static void v_el(JNIEnv*, jlong l) {}
+  static void v_et(JNIEnv*, jstring) {}
+  static jobject o_none() { return nullptr; }
+  static void bad_noref_jint_norm(JNIEnv*, jclass, jint&) {}
+  static void bad_noref_jint_crit(jint&) {}
+
+  // Good depending on the context:
+
+  // CriticalNative
+  static void empty_fn() {}
+  static jint int_fn() { return 0; }
+
+  static void v_() {}
+  static void v_vol_i(volatile jint) {}
+  static void v_const_i(const jint) {}
+  static void v_i(jint) {}
+  static void v_l(jlong) {}
+  static void v_lib(jlong, jint, jboolean) {}
+  static jshort s_lib(jlong, jint, jboolean) { return 0; }
+
+  // Normal or FastNative.
+  static void v_eo(JNIEnv*, jobject) {}
+  static void v_eoo(JNIEnv*, jobject, jobject) {}
+  static void v_ek(JNIEnv*, jclass) {}
+  static void v_eolib(JNIEnv*, jobject, jlong, jint, jboolean) {}
+  static jshort s_eolAibA(JNIEnv*, jobject, jlongArray, jint, jbooleanArray) { return 0; }
+
+#define DEC_TEST_FN_IMPL(name, ret_t, ...) \
+  static ret_t name (__VA_ARGS__) { return TestReturnAnything{}; }
+
+#define DEC_TEST_FN(name, correct, ret_t, ...) \
+  DEC_TEST_FN_IMPL(normal_ ## name, ret_t, JNIEnv*, jobject, __VA_ARGS__) \
+  DEC_TEST_FN_IMPL(normal2_ ## name, ret_t, JNIEnv*, jclass, __VA_ARGS__) \
+  DEC_TEST_FN_IMPL(critical_ ## name, ret_t, __VA_ARGS__)
+
+#define DEC_TEST_FN0(name, correct, ret_t) \
+  DEC_TEST_FN_IMPL(normal_ ## name, ret_t, JNIEnv*, jobject) \
+  DEC_TEST_FN_IMPL(normal2_ ## name, ret_t, JNIEnv*, jclass) \
+  DEC_TEST_FN_IMPL(critical_ ## name, ret_t)
+
+#define JNI_TEST_FN(FN, FN0) \
+  FN0(a0,CRITICAL,void) \
+  FN0(a ,CRITICAL,jboolean) \
+  FN0(a1,CRITICAL,jbyte) \
+  FN0(g, CRITICAL,jchar) \
+  FN0(c, CRITICAL,jshort) \
+  FN0(b, CRITICAL,jint) \
+  FN0(f, CRITICAL,jlong) \
+  FN0(d, CRITICAL,jfloat) \
+  FN0(e, CRITICAL,jdouble) \
+  FN0(f2,NORMAL  ,jobject) \
+  FN0(f3,NORMAL  ,jclass) \
+  FN0(fr,NORMAL  ,jstring) \
+  FN0(fa,NORMAL  ,jarray) \
+  FN0(fb,NORMAL  ,jobjectArray) \
+  FN0(fc,NORMAL  ,jbooleanArray) \
+  FN0(fd,NORMAL  ,jcharArray) \
+  FN0(fe,NORMAL  ,jshortArray) \
+  FN0(ff,NORMAL  ,jintArray) \
+  FN0(fg,NORMAL  ,jlongArray) \
+  FN0(fk,NORMAL  ,jfloatArray) \
+  FN0(fi,NORMAL  ,jdoubleArray) \
+  FN0(fl,NORMAL  ,jthrowable) \
+  FN(aa, CRITICAL,jboolean,jboolean) \
+  FN(ax, CRITICAL,jbyte,jbyte) \
+  FN(ag, CRITICAL,jchar,jchar) \
+  FN(ac, CRITICAL,jshort,jshort) \
+  FN(ac2,CRITICAL,jshort,jshort,jchar) \
+  FN(ab, CRITICAL,jint,jint) \
+  FN(af, CRITICAL,jlong,jlong) \
+  FN(ad, CRITICAL,jfloat,jfloat) \
+  FN(ae, CRITICAL,jdouble,jdouble) \
+  FN(af2,NORMAL  ,jobject,jobject) \
+  FN(af3,NORMAL  ,jclass,jclass) \
+  FN(afr,NORMAL  ,jstring,jstring) \
+  FN(afa,NORMAL  ,jarray,jarray) \
+  FN(afb,NORMAL  ,jobjectArray,jobjectArray) \
+  FN(afc,NORMAL  ,jbooleanArray,jbooleanArray) \
+  FN(afd,NORMAL  ,jcharArray,jcharArray) \
+  FN(afe,NORMAL  ,jshortArray,jshortArray) \
+  FN(aff,NORMAL  ,jintArray,jintArray) \
+  FN(afg,NORMAL  ,jlongArray,jlongArray) \
+  FN(afk,NORMAL  ,jfloatArray,jfloatArray) \
+  FN(afi,NORMAL  ,jdoubleArray,jdoubleArray) \
+  FN(agi,NORMAL  ,jdoubleArray,jdoubleArray,jobject) \
+  FN(afl,NORMAL  ,jthrowable,jthrowable) \
+  \
+  FN0(z0,ILLEGAL ,JNIEnv*) \
+  FN(z1, ILLEGAL ,void, JNIEnv*) \
+  FN(z2, ILLEGAL ,JNIEnv*, JNIEnv*) \
+  FN(z3, ILLEGAL ,void, void*) \
+  FN0(z4,ILLEGAL ,void*) \
+
+#define JNI_TEST_FN_BOTH(x) JNI_TEST_FN(x,x)
+
+// we generate a return statement because some functions are non-void.
+// disable the useless warning about returning from a non-void function.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wreturn-type"
+  JNI_TEST_FN(DEC_TEST_FN, DEC_TEST_FN0);
+#pragma clang diagnostic pop
+
+  // TODO: probably should be an x-macro table
+  // and that way we can add critical/normal to it as well
+  // and also the type descriptor, and reuse this for multiple tests.
+
+#pragma clang diagnostic pop
+};
+// Note: Using function-local structs does not work.
+// Template parameters must have linkage, which function-local structs lack.
+
+TEST(JniSafeRegisterNativeMethods, FunctionTypes) {
+  using namespace nativehelper::detail;
+  // The exact error messages are not tested but they would be seen in the compiler
+  // stack trace when used from a constexpr context.
+
+#define IS_VALID_JNI_FUNCTION_TYPE(native_kind, func) (IsValidJniFunctionType<native_kind, decltype(func), (func)>())
+#define IS_VALID_NORMAL_JNI_FUNCTION_TYPE(func) IS_VALID_JNI_FUNCTION_TYPE(kNormalNative, func)
+#define IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(func) IS_VALID_JNI_FUNCTION_TYPE(kCriticalNative, func)
+
+#define EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(func)                      \
+    do {                                                            \
+       EXPECT_FALSE(IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(func));    \
+       EXPECT_FALSE(IS_VALID_NORMAL_JNI_FUNCTION_TYPE(func));      \
+    } while (false)
+
+#define EXPECT_EITHER_JNI_FUNCTION_TYPE(func)                       \
+    do {                                                            \
+       EXPECT_TRUE(IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(func));     \
+       EXPECT_TRUE(IS_VALID_NORMAL_JNI_FUNCTION_TYPE(func));       \
+    } while (false)
+
+#define EXPECT_NORMAL_JNI_FUNCTION_TYPE(func)                       \
+    do {                                                            \
+       EXPECT_FALSE(IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(func));    \
+       EXPECT_TRUE(IS_VALID_NORMAL_JNI_FUNCTION_TYPE(func));       \
+    } while (false)
+
+#define EXPECT_CRITICAL_JNI_FUNCTION_TYPE(func)                    \
+  do {                                                             \
+     EXPECT_TRUE(IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(func));      \
+     EXPECT_FALSE(IS_VALID_NORMAL_JNI_FUNCTION_TYPE(func));       \
+  } while (false)
+
+  {
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_cptr);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_ret_ptr);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_ret_env);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_wrongplace_env);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_wrongplace_env2);
+
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::empty_fn);
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(test_jni::empty_fn);
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::int_fn);
+
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::v_);
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::v_vol_i);
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::v_const_i);
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::v_i);
+    EXPECT_CRITICAL_JNI_FUNCTION_TYPE(TestJni::v_l);
+
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::v_e);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::v_ei);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::v_el);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::v_et);
+
+    EXPECT_NORMAL_JNI_FUNCTION_TYPE(TestJni::v_eo);
+    EXPECT_NORMAL_JNI_FUNCTION_TYPE(TestJni::v_ek);
+
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::o_none);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_noref_jint_norm);
+    EXPECT_ILLEGAL_JNI_FUNCTION_TYPE(TestJni::bad_noref_jint_crit);
+  }
+
+  enum class TestJniKind {
+    ILLEGAL,
+    NORMAL,
+    CRITICAL
+  };
+
+  // ILLEGAL signatures are always illegal.
+  bool kExpected_ILLEGAL_against_NORMAL = false;
+  bool kExpected_ILLEGAL_against_CRITICAL = false;
+  // NORMAL signatures are only legal for Normal JNI.
+  bool kExpected_NORMAL_against_NORMAL = true;
+  bool kExpected_NORMAL_against_CRITICAL = false;
+  // CRITICAL signatures are legal for both Normal+Critical JNI.
+  bool kExpected_CRITICAL_against_CRITICAL = true;
+  bool kExpected_CRITICAL_against_NORMAL = true;
+  // Note that we munge normal and critical type signatures separately
+  // and that a normal_ prefixed is always a bad critical signature,
+  // and a critical_ prefixed signature is always a bad normal signature.
+  // See JNI_TEST_FN_MAKE_TEST for the implementation of this logic.
+
+#undef EXPECTED_FOR
+#define EXPECTED_FOR(jni_kind, context) \
+  (kExpected_ ## jni_kind ## _against_ ## context)
+
+  {
+#define JNI_TEST_FN_MAKE_TEST(name, jni_kind, ...) \
+     do {                                                            \
+       EXPECT_EQ(EXPECTED_FOR(jni_kind, NORMAL),                     \
+                 IS_VALID_NORMAL_JNI_FUNCTION_TYPE(TestJni::normal_ ## name));  \
+       EXPECT_FALSE(IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(TestJni::normal_ ## name)); \
+       EXPECT_EQ(EXPECTED_FOR(jni_kind, NORMAL),                     \
+                 IS_VALID_NORMAL_JNI_FUNCTION_TYPE(TestJni::normal2_ ## name)); \
+       EXPECT_FALSE(IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(TestJni::normal2_ ## name)); \
+       EXPECT_EQ(EXPECTED_FOR(jni_kind, CRITICAL),                   \
+                 IS_VALID_CRITICAL_JNI_FUNCTION_TYPE(TestJni::critical_ ## name)); \
+       EXPECT_FALSE(IS_VALID_NORMAL_JNI_FUNCTION_TYPE(TestJni::critical_ ## name)); \
+    } while (false);
+
+    JNI_TEST_FN_BOTH(JNI_TEST_FN_MAKE_TEST);
+  }
+}
+
+#define EXPECT_CONSTEXPR_EQ(lhs, rhs) \
+ { constexpr auto lhs_val = (lhs);    \
+   constexpr auto rhs_val = (rhs);    \
+   EXPECT_EQ(lhs_val, rhs_val) << "LHS: " << #lhs << ", RHS: " << #rhs; \
+ }
+
+TEST(JniSafeRegisterNativeMethods, FunctionTypeDescriptorConversion) {
+  using namespace nativehelper::detail;
+  {
+    constexpr auto cvrt = MaybeMakeReifiedJniSignature<kCriticalNative,
+                                                       decltype(TestJni::v_i),
+                                                       TestJni::v_i>();
+    EXPECT_TRUE(cvrt.has_value());
+    EXPECT_CONSTEXPR_EQ(2u, cvrt->max_size);
+    EXPECT_CONSTEXPR_EQ(1u, cvrt->args.size());
+    EXPECT_STRINGIFY_EQ("args={jint}, ret=void", cvrt.value());
+  }
+
+  {
+    constexpr auto cvrt = MaybeMakeReifiedJniSignature<kNormalNative,
+                                                       decltype(TestJni::v_i),
+                                                       TestJni::v_i>();
+    EXPECT_FALSE(cvrt.has_value());
+  }
+
+  {
+    constexpr auto cvrt = MaybeMakeReifiedJniSignature<kNormalNative,
+                                                       decltype(TestJni::normal_agi),
+                                                       TestJni::normal_agi>();
+    EXPECT_TRUE(cvrt.has_value());
+    EXPECT_EQ(2u, cvrt->args.size());
+    EXPECT_STRINGIFY_EQ("args={jdoubleArray,jobject}, ret=jdoubleArray", cvrt.value());
+  }
+
+  {
+    constexpr auto cvrt = MaybeMakeReifiedJniSignature<kCriticalNative,
+                                                       decltype(TestJni::critical_ac2),
+                                                       TestJni::critical_ac2>();
+    EXPECT_TRUE(cvrt.has_value());
+    EXPECT_EQ(2u, cvrt->args.size());
+    EXPECT_STRINGIFY_EQ("args={jshort,jchar}, ret=jshort", cvrt.value());
+  }
+
+  // TODO: use JNI_TEST_FN to generate these tests automatically.
+}
+
+struct test_function_traits {
+  static int int_returning_function() { return 0; }
+};
+
+template <typename T>
+struct apply_return_type {
+  constexpr int operator()() const {
+    return sizeof(T) == sizeof(int);
+  }
+};
+
+#define FN_ARGS_PAIR(fn) decltype(fn), (fn)
+
+TEST(JniSafeRegisterNativeMethods, FunctionTraits) {
+  using namespace nativehelper::detail;
+  using traits_for_int_ret = FunctionTypeMetafunction<FN_ARGS_PAIR(test_function_traits::int_returning_function)>;
+  int applied = traits_for_int_ret::map_return<apply_return_type>();
+  EXPECT_EQ(1, applied);
+
+  auto arr = traits_for_int_ret::map_args<apply_return_type>();
+  EXPECT_EQ(0u, arr.size());
+}
+
+struct IntHolder {
+  int value;
+};
+
+constexpr int GetTestValue(const IntHolder& i) {
+  return i.value;
+}
+
+constexpr int GetTestValue(int i) {
+  return i;
+}
+
+template <typename T, size_t kMaxSize>
+constexpr size_t SumUpVector(const nativehelper::detail::ConstexprVector<T, kMaxSize>& vec) {
+  size_t s = 0;
+  for (const T& elem : vec) {
+    s += static_cast<size_t>(GetTestValue(elem));
+  }
+  return s;
+}
+
+template <typename T>
+constexpr auto make_test_int_vector() {
+  using namespace nativehelper::detail;
+  ConstexprVector<T, 5> vec_int;
+  vec_int.push_back(T{1});
+  vec_int.push_back(T{2});
+  vec_int.push_back(T{3});
+  vec_int.push_back(T{4});
+  vec_int.push_back(T{5});
+  return vec_int;
+}
+
+TEST(JniSafeRegisterNativeMethods, ConstexprVector) {
+  using namespace nativehelper::detail;
+  {
+    constexpr ConstexprVector<IntHolder, 5> vec_int = make_test_int_vector<IntHolder>();
+    constexpr size_t the_sum = SumUpVector(vec_int);
+    EXPECT_EQ(15u, the_sum);
+  }
+
+  {
+    constexpr ConstexprVector<int, 5> vec_int = make_test_int_vector<int>();
+    constexpr size_t the_sum = SumUpVector(vec_int);
+    EXPECT_EQ(15u, the_sum);
+  }
+}
+
+// Need this intermediate function to make a JniDescriptorNode from a string literal.
+// C++ doesn't do implicit conversion through two+ type constructors.
+constexpr nativehelper::detail::JniDescriptorNode MakeNode(
+    nativehelper::detail::ConstexprStringView str) {
+  return nativehelper::detail::JniDescriptorNode{str};
+}
+
+#define EXPECT_EQUALISH_JNI_DESCRIPTORS_IMPL(user_desc, derived, cond) \
+  do { \
+    constexpr bool res = CompareJniDescriptorNodeErased(MakeNode(user_desc), ReifiedJniTypeTrait::Reify<derived>()); \
+    (void)res; \
+    EXPECT_ ## cond(CompareJniDescriptorNodeErased(MakeNode(user_desc), ReifiedJniTypeTrait::Reify<derived>())); \
+  } while (0);
+
+#define EXPECT_EQUALISH_JNI_DESCRIPTORS(user_desc, derived_desc) \
+  EXPECT_EQUALISH_JNI_DESCRIPTORS_IMPL(user_desc, derived_desc, TRUE)
+
+#define EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS(user_desc, derived_desc) \
+  EXPECT_EQUALISH_JNI_DESCRIPTORS_IMPL(user_desc, derived_desc, FALSE)
+
+TEST(JniSafeRegisterNativeMethods, CompareJniDescriptorNodeErased) {
+  using namespace nativehelper::detail;
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("V", void);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("V", jint);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Z", jboolean);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Z", void);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Z", jobject);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("J", jlong);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("J", jobject);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("J", jthrowable);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("J", jint);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/String;", jstring);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Class;", jclass);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Object;", jobject);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Integer;", jobject);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("[Z", jthrowable);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("[Z", jobjectArray);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Integer;", jintArray);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Integer;", jarray);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Integer;", jarray);
+
+  // Stricter checks.
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Object;", jobjectArray);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/String;", jobject);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Class;", jobject);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("[Z", jobject);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("[Ljava/lang/Object;", jobject);
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Object;", jarray);
+
+  // Permissive checks that are weaker than normal.
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Exception;", jobject);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Error;", jobject);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("[Z", jarray);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("[I", jarray);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("[[Z", jarray);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("[[Ljava/lang/Object;", jarray);
+
+  // jthrowable-related checks.
+  EXPECT_NOT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Throwable;", jobject);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Throwable;", jthrowable);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Exception;", jthrowable);
+  EXPECT_EQUALISH_JNI_DESCRIPTORS("Ljava/lang/Error;", jthrowable);
+}
+
+#define EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH(type_desc, type) \
+  do { \
+    constexpr auto res = ReifiedJniTypeTrait::MostSimilarTypeDescriptor(type_desc); \
+    EXPECT_TRUE((ReifiedJniTypeTrait::MostSimilarTypeDescriptor(type_desc)).has_value());\
+    if (res.has_value()) EXPECT_EQ(ReifiedJniTypeTrait::Reify<type>(), res.value());  \
+  } while (false)
+
+#define EXPECT_SIMILAR_TYPE_DESCRIPTOR_NO_MATCH(type_desc) \
+  do { \
+    auto res = ReifiedJniTypeTrait::MostSimilarTypeDescriptor(type_desc); \
+    EXPECT_FALSE(res.has_value());                                        \
+  } while (false)
+
+#define JNI_TYPE_TRAIT_MUST_BE_SAME_FN(type_name, type_desc, ...)              \
+  /* skip jarray because it aliases Ljava/lang/Object; */                      \
+  do {                                                                         \
+    constexpr auto str_type_name = ConstexprStringView(#type_name);          \
+    if (str_type_name != "jarray" && str_type_name != "JNIEnv*") {             \
+      EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH(type_desc, type_name);              \
+    }                                                                          \
+  } while(false);
+
+TEST(JniSafeRegisterNativeMethods, MostSimilarTypeDescriptor) {
+  using namespace nativehelper::detail;
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH("Z", jboolean);
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH("[[I", jobjectArray);
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH("[[Z", jobjectArray);
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH("[Ljava/lang/String;", jobjectArray);
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_MATCH("[Ljava/lang/Integer;", jobjectArray);
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_NO_MATCH("illegal");
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_NO_MATCH("?");
+  EXPECT_SIMILAR_TYPE_DESCRIPTOR_NO_MATCH("");
+
+  DEFINE_JNI_TYPE_TRAIT(JNI_TYPE_TRAIT_MUST_BE_SAME_FN);
+}
+
+#define ENFORCE_CONSTEXPR(expr) \
+  static_assert(__builtin_constant_p(expr), "Expression must be constexpr")
+
+#define EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION_IMPL(cond, native_kind, func, desc) \
+  do {                                                                        \
+    ENFORCE_CONSTEXPR((MatchJniDescriptorWithFunctionType<                    \
+        native_kind,                                                          \
+        decltype(func),                                                       \
+        func,                                                                 \
+        sizeof(desc)>(desc)));                                                \
+    EXPECT_ ## cond((MatchJniDescriptorWithFunctionType<                      \
+        native_kind,                                                          \
+        decltype(func),                                                       \
+        func,                                                                 \
+        sizeof(desc)>(desc)));                                                \
+  } while(0)
+
+#define EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(native_kind, func, desc) \
+    EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION_IMPL(TRUE, native_kind, func, desc)
+
+#define EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(native_kind, func, desc) \
+    EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION_IMPL(FALSE, native_kind, func, desc)
+
+TEST(JniSafeRegisterNativeMethods, MatchJniDescriptorWithFunctionType) {
+  using namespace nativehelper::detail;
+  // Bad C++ signature.
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::bad_cptr, "()V");
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::bad_cptr, "()V");
+
+  // JNI type descriptor is not legal (by itself).
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::v_, "BAD");
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::v_eo, "BAD");
+
+  // Number of parameters in signature vs C++ function does not match.
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::v_i, "()V");
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::v_eoo, "()V");
+
+  // Return types don't match.
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::v_, "()Z");
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kFastNative, TestJni::v_eo, "()Z");
+
+  // Argument types don't match.
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::v_i, "(Z)V");
+  EXPECT_NO_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::v_eoo, "(Ljava/lang/Class;)V");
+
+  // OK.
+  EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::v_i, "(I)V");
+  EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::v_eoo, "(Ljava/lang/Object;)V");
+
+  EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::v_lib, "(JIZ)V");
+  EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::v_eolib, "(JIZ)V");
+  EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kCriticalNative, TestJni::s_lib, "(JIZ)S");
+  EXPECT_MATCH_JNI_DESCRIPTOR_AGAINST_FUNCTION(kNormalNative, TestJni::s_eolAibA, "([JI[Z)S");
+}
+
+TEST(JniSafeRegisterNativeMethods, Infer) {
+  using namespace nativehelper::detail;
+  {
+    using Infer_v_eolib_t = InferJniDescriptor<kNormalNative,
+                                               decltype(TestJni::v_eolib),
+                                               TestJni::v_eolib>;
+    EXPECT_CONSTEXPR_EQ(6u, Infer_v_eolib_t::kMaxStringSize);
+    std::string x = Infer_v_eolib_t::GetStringAtRuntime();
+    EXPECT_STRINGIFY_EQ("(JIZ)V", x.c_str());
+  }
+
+  {
+    using Infer_v_eolib_t = InferJniDescriptor<kNormalNative,
+                                               decltype(TestJni::s_eolAibA),
+                                               TestJni::s_eolAibA>;
+    EXPECT_STRINGIFY_EQ("args={[J,I,[Z}, ret=S", Infer_v_eolib_t::FromFunctionType().value());
+    EXPECT_CONSTEXPR_EQ(8u, Infer_v_eolib_t::kMaxStringSize);
+    std::string x = Infer_v_eolib_t::GetStringAtRuntime();
+    EXPECT_STRINGIFY_EQ("([JI[Z)S", x.c_str());
+  }
+}
+
+// Test the macro definition only. See other tests above for signature-match testing.
+TEST(JniSafeRegisterNativeMethods, MakeCheckedJniNativeMethod) {
+  // Ensure the temporary variables don't conflict with other local vars of same name.
+  JNINativeMethod tmp_native_method;  // shadow test.
+  (void) tmp_native_method;
+  bool is_signature_valid = true;  // shadow test.
+  (void) is_signature_valid;
+
+  // Ensure it works with critical.
+  {
+    JNINativeMethod m =
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kCriticalNative,
+                                       "v_lib",
+                                       "(JIZ)V",
+                                       TestJni::v_lib);
+    (void)m;
+  }
+
+  // Ensure it works with normal.
+  {
+    JNINativeMethod m =
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kNormalNative,
+                                       "v_eolib",
+                                       "(JIZ)V",
+                                       TestJni::v_eolib);
+    (void)m;
+  }
+
+  // Make sure macros properly expand inside of an array.
+  {
+    JNINativeMethod m_array[] = {
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kCriticalNative,
+                                       "v_lib",
+                                       "(JIZ)V",
+                                       TestJni::v_lib),
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kNormalNative,
+                                       "v_eolib",
+                                       "(JIZ)V",
+                                       TestJni::v_eolib),
+    };
+    (void)m_array;
+  }
+  {
+    JNINativeMethod m_array_direct[] {
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kCriticalNative,
+                                       "v_lib",
+                                       "(JIZ)V",
+                                       TestJni::v_lib),
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kNormalNative,
+                                       "v_eolib",
+                                       "(JIZ)V",
+                                       TestJni::v_eolib),
+    };
+    (void)m_array_direct;
+  }
+
+}
+
+static auto sTestCheckedAtFileScope =
+        MAKE_CHECKED_JNI_NATIVE_METHOD(kCriticalNative,
+                                       "v_lib",
+                                       "(JIZ)V",
+                                       TestJni::v_lib);
+
+static auto sTestInferredAtFileScope =
+        MAKE_INFERRED_JNI_NATIVE_METHOD(kCriticalNative,
+                                       "v_lib",
+                                       TestJni::v_lib);
+
+TEST(JniSafeRegisterNativeMethods, TestInferredJniNativeMethod) {
+  (void) sTestCheckedAtFileScope;
+  (void) sTestInferredAtFileScope;
+
+  // Ensure it works with critical.
+  {
+    JNINativeMethod m =
+        MAKE_INFERRED_JNI_NATIVE_METHOD(kCriticalNative,
+                                       "v_lib",
+                                       TestJni::v_lib);
+    (void)m;
+  }
+
+  // Ensure it works with normal.
+  {
+    JNINativeMethod m =
+        MAKE_INFERRED_JNI_NATIVE_METHOD(kNormalNative,
+                                       "v_eolib",
+                                       TestJni::v_eolib);
+    (void)m;
+  }
+}
+
+static void TestJniMacros_v_lib(jlong, jint, jboolean) {}
+static void TestJniMacros_v_lib_od(jlong, jint, jboolean) {}
+static void TestJniMacros_v_eolib(JNIEnv*, jobject, jlong, jint, jboolean) {}
+static void TestJniMacros_v_eolib_od(JNIEnv*, jobject, jlong, jint, jboolean) {}
+
+TEST(JniSafeRegisterNativeMethods, JniMacros) {
+  JNINativeMethod tmp_native_method;  // shadow variable check.
+  (void)tmp_native_method;
+  using Infer_t = int;  // shadow using check.
+  Infer_t unused;
+  (void)unused;
+
+  MAKE_JNI_CRITICAL_NATIVE_METHOD("v_lib", "(JIZ)V", TestJniMacros_v_lib);
+  MAKE_JNI_CRITICAL_NATIVE_METHOD_AUTOSIG("v_lib", TestJniMacros_v_lib);
+  CRITICAL_NATIVE_METHOD(TestJniMacros, v_lib, "(JIZ)V");
+  OVERLOADED_CRITICAL_NATIVE_METHOD(TestJniMacros, v_lib, "(JIZ)V", v_lib_od);
+  CRITICAL_NATIVE_METHOD_AUTOSIG(TestJniMacros, v_lib);
+
+  MAKE_JNI_FAST_NATIVE_METHOD("v_eolib", "(JIZ)V", TestJniMacros_v_eolib);
+  MAKE_JNI_FAST_NATIVE_METHOD_AUTOSIG("v_eolib", TestJniMacros_v_eolib);
+  FAST_NATIVE_METHOD(TestJniMacros, v_eolib, "(JIZ)V");
+  OVERLOADED_FAST_NATIVE_METHOD(TestJniMacros, v_eolib, "(JIZ)V", v_eolib_od);
+  FAST_NATIVE_METHOD_AUTOSIG(TestJniMacros, v_eolib);
+
+  MAKE_JNI_NATIVE_METHOD("v_eolib", "(JIZ)V", TestJniMacros_v_eolib);
+  MAKE_JNI_NATIVE_METHOD_AUTOSIG("v_eolib", TestJniMacros_v_eolib);
+  NATIVE_METHOD(TestJniMacros, v_eolib, "(JIZ)V");
+  OVERLOADED_NATIVE_METHOD(TestJniMacros, v_eolib, "(JIZ)V", v_eolib_od);
+  NATIVE_METHOD_AUTOSIG(TestJniMacros, v_eolib);
+
+  _NATIVEHELPER_JNI_MAKE_METHOD_OLD(kNormalNative, "v_eolib", "(JIZ)V", TestJniMacros_v_eolib);
+  tmp_native_method =
+      _NATIVEHELPER_JNI_MAKE_METHOD_OLD(kNormalNative, "v_eolib", "(JIZ)V", TestJniMacros_v_eolib);
+}