pw_polyfill: Update language feature support
- Support using C++20's consteval and constinit keywords in older C++
versions.
- Support C++ language features through an implicitly included header.
This allows all C++ code to use modern-style static_assert and the
consteval or constinit keywords without including any headers.
Change-Id: I53a05ab6d6cdd801996498e7f93c5e68bf14d315
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/15662
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_polyfill/language_features.h b/pw_polyfill/language_features.h
new file mode 100644
index 0000000..0a0e5cb
--- /dev/null
+++ b/pw_polyfill/language_features.h
@@ -0,0 +1,69 @@
+// Copyright 2020 The Pigweed Authors
+//
+// 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
+//
+// https://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.
+
+// This file provides adapters for newer C++ language features so that they can
+// be used in older versions of C++ (though the code will not function exactly
+// the same). This file is not on an include path and is intended to be used
+// with -include when compiling C++.
+//
+// pw_polyfill/language_feature_macros.h provides macro wrappers for a few
+// specific uses of modern C++ keywords.
+#pragma once
+
+// C++11 is required for the features in this header.
+#if __cplusplus >= 201103L
+
+// If consteval is not supported, use constexpr. This does not guarantee
+// compile-time execution, but works equivalently in constant expressions.
+#ifndef __cpp_consteval
+#define consteval constexpr
+#endif // __cpp_consteval
+
+// If constinit is not supported, use a compiler attribute or omit it. If
+// omitted, the compiler may still constant initialize the variable, but there
+// is no guarantee.
+#ifndef __cpp_constinit
+#ifdef __clang__
+#define constinit [[clang::require_constant_initialization]]
+#else
+#define constinit
+#endif // __clang__
+#endif // __cpp_constinit
+
+// This is an adapter for supporting static_assert with a single argument in
+// C++11 or C++14. Macros don't correctly parse commas in template expressions,
+// so the static_assert arguments are passed to an overloaded C++ function. The
+// full stringified static_assert arguments are used as the message.
+#if __cpp_static_assert < 201411L
+
+#define static_assert(...) \
+ static_assert(::pw::polyfill::internal::StaticAssertExpression(__VA_ARGS__), \
+ #__VA_ARGS__)
+
+namespace pw {
+namespace polyfill {
+namespace internal {
+
+constexpr bool StaticAssertExpression(bool expression) { return expression; }
+
+constexpr bool StaticAssertExpression(bool expression, const char*) {
+ return expression;
+}
+
+} // namespace internal
+} // namespace polyfill
+} // namespace pw
+
+#endif // __cpp_static_assert < 201411L
+#endif // __cplusplus >= 201103L