test/SemaCXX/cxx1y-deduced-return-type.cpp

// RUN: %clang_cc1 -std=c++1y -verify -fsyntax-only %s

auto f(); // expected-note {{previous}}
int f(); // expected-error {{differ only in their return type}}

auto &g();
auto g() -> auto &;

auto h() -> auto *;
auto *h();

struct Conv1 {
  operator auto(); // expected-note {{declared here}}
} conv1;
int conv1a = conv1; // expected-error {{function 'operator auto' with deduced return type cannot be used before it is defined}}
// expected-error@-1 {{no viable conversion}}
Conv1::operator auto() { return 123; }
int conv1b = conv1;
int conv1c = conv1.operator auto();
int conv1d = conv1.operator int(); // expected-error {{no member named 'operator int'}}

struct Conv2 {
  operator auto() { return 0; }  // expected-note 2{{previous}}
  operator auto() { return 0.; } // expected-error {{cannot be redeclared}} expected-error {{redefinition of 'operator auto'}}
};

struct Conv3 {
  operator auto() { int *p = nullptr; return p; }  // expected-note {{candidate}}
  operator auto*() { int *p = nullptr; return p; } // expected-note {{candidate}}
} conv3;
int *conv3a = conv3; // expected-error {{ambiguous}}
int *conv3b = conv3.operator auto();
int *conv3c = conv3.operator auto*();

template<typename T>
struct Conv4 {
  operator auto() { return T(); }
};
Conv4<int> conv4int;
int conv4a = conv4int;
int conv4b = conv4int.operator auto();

auto a();
auto a() { return 0; }
using T = decltype(a());
using T = int;
auto a(); // expected-note {{previous}}
using T = decltype(a());
auto *a(); // expected-error {{differ only in their return type}}

auto b(bool k) {
  if (k)
    return "hello";
  return "goodbye";
}

auto *ptr_1() {
  return 100; // expected-error {{cannot deduce return type 'auto *' from returned value of type 'int'}}
}

const auto &ref_1() {
  return 0; // expected-warning {{returning reference to local temporary}}
}

auto init_list() {
  return { 1, 2, 3 }; // expected-error {{cannot deduce return type from initializer list}}
}

auto fwd_decl(); // expected-note 2{{here}}

int n = fwd_decl(); // expected-error {{function 'fwd_decl' with deduced return type cannot be used before it is defined}}
int k = sizeof(fwd_decl()); // expected-error {{used before it is defined}}

auto fac(int n) {
  if (n <= 2)
    return n;
  return n * fac(n-1); // ok
}

auto fac_2(int n) { // expected-note {{declared here}}
  if (n > 2)
    return n * fac_2(n-1); // expected-error {{cannot be used before it is defined}}
  return n;
}

auto void_ret() {}
using Void = void;
using Void = decltype(void_ret());

auto &void_ret_2() {} // expected-error {{cannot deduce return type 'auto &' for function with no return statements}}
const auto void_ret_3() {} // ok, return type 'const void' is adjusted to 'void'

const auto void_ret_4() {
  if (false)
    return void();
  if (false)
    return;
  return 0; // expected-error {{'auto' in return type deduced as 'int' here but deduced as 'void' in earlier return statement}}
}

namespace Templates {
  template<typename T> auto f1() {
    return T() + 1;
  }
  template<typename T> auto &f2(T &&v) { return v; }
  int a = f1<int>();
  const int &b = f2(0);
  double d;
  float &c = f2(0.0); // expected-error {{non-const lvalue reference to type 'float' cannot bind to a value of unrelated type 'double'}}

  template<typename T> auto fwd_decl(); // expected-note {{declared here}}
  int e = fwd_decl<int>(); // expected-error {{cannot be used before it is defined}}
  template<typename T> auto fwd_decl() { return 0; }
  int f = fwd_decl<int>();
  template<typename T> auto fwd_decl();
  int g = fwd_decl<char>();

  auto (*p)() = f1; // expected-error {{incompatible initializer}}
  auto (*q)() = f1<int>; // ok

  typedef decltype(f2(1.2)) dbl; // expected-note {{previous}}
  typedef float dbl; // expected-error {{typedef redefinition with different types ('float' vs 'decltype(f2(1.2))' (aka 'double &'))}}

  extern template auto fwd_decl<double>();
  int k1 = fwd_decl<double>();
  extern template int fwd_decl<char>(); // expected-error {{does not refer to a function template}}
  int k2 = fwd_decl<char>();

  template<typename T> auto instantiate() { T::error; } // expected-error {{has no members}}
  extern template auto instantiate<int>(); // ok
  int k = instantiate<int>(); // expected-note {{in instantiation of}}
  template<> auto instantiate<char>() {} // ok
  template<> void instantiate<double>() {} // expected-error {{no function template matches}}

  template<typename T> auto arg_single() { return 0; }
  template<typename T> auto arg_multi() { return 0l; }
  template<typename T> auto arg_multi(int) { return "bad"; }
  template<typename T> struct Outer {
    static auto arg_single() { return 0.f; }
    static auto arg_multi() { return 0.; }
    static auto arg_multi(int) { return "bad"; }
  };
  template<typename T> T &take_fn(T (*p)());

  int &check1 = take_fn(arg_single); // expected-error {{no matching}} expected-note@-2 {{couldn't infer}}
  int &check2 = take_fn(arg_single<int>);
  int &check3 = take_fn<int>(arg_single); // expected-error {{no matching}} expected-note@-4{{no overload of 'arg_single'}}
  int &check4 = take_fn<int>(arg_single<int>);
  long &check5 = take_fn(arg_multi); // expected-error {{no matching}} expected-note@-6 {{couldn't infer}}
  long &check6 = take_fn(arg_multi<int>);
  long &check7 = take_fn<long>(arg_multi); // expected-error {{no matching}} expected-note@-8{{no overload of 'arg_multi'}}
  long &check8 = take_fn<long>(arg_multi<int>);

  float &mem_check1 = take_fn(Outer<int>::arg_single);
  float &mem_check2 = take_fn<float>(Outer<char>::arg_single);
  double &mem_check3 = take_fn(Outer<long>::arg_multi);
  double &mem_check4 = take_fn<double>(Outer<double>::arg_multi);

  namespace Deduce1 {
    template<typename T> auto f() { return 0; } // expected-note {{candidate}}
    template<typename T> void g(T(*)()); // expected-note 2{{candidate}}
    void h() {
      auto p = f<int>;
      auto (*q)() = f<int>;
      int (*r)() = f; // expected-error {{does not match}}
      g(f<int>);
      g<int>(f); // expected-error {{no matching function}}
      g(f); // expected-error {{no matching function}}
    }
  }

  namespace Deduce2 {
    template<typename T> auto f(int) { return 0; } // expected-note {{candidate}}
    template<typename T> void g(T(*)(int)); // expected-note 2{{candidate}}
    void h() {
      auto p = f<int>;
      auto (*q)(int) = f<int>;
      int (*r)(int) = f; // expected-error {{does not match}}
      g(f<int>);
      g<int>(f); // expected-error {{no matching function}}
      g(f); // expected-error {{no matching function}}
    }
  }

  namespace Deduce3 {
    template<typename T> auto f(T) { return 0; }
    template<typename T> void g(T(*)(int)); // expected-note {{couldn't infer}}
    void h() {
      auto p = f<int>;
      auto (*q)(int) = f<int>;
      int (*r)(int) = f; // ok
      g(f<int>);
      g<int>(f); // ok
      g(f); // expected-error {{no matching function}}
    }
  }

  namespace DeduceInDeducedReturnType {
    template<typename T, typename U> auto f() -> auto (T::*)(U) {
      int (T::*result)(U) = nullptr;
      return result;
    }
    struct S {};
    int (S::*(*p)())(double) = f;
    int (S::*(*q)())(double) = f<S, double>;
  }
}

auto fwd_decl_using();
namespace N { using ::fwd_decl_using; }
auto fwd_decl_using() { return 0; }
namespace N { int k = N::fwd_decl_using(); }

namespace OverloadResolutionNonTemplate {
  auto f();
  auto f(int); // expected-note {{here}}

  int &g(int (*f)()); // expected-note {{not viable: no overload of 'f' matching 'int (*)()'}}
  char &g(int (*f)(int)); // expected-note {{not viable: no overload of 'f' matching 'int (*)(int)'}}

  int a = g(f); // expected-error {{no matching function}}

  auto f() { return 0; }

  // FIXME: It's not completely clear whether this should be ill-formed.
  int &b = g(f); // expected-error {{used before it is defined}}

  auto f(int) { return 0.0; }

  int &c = g(f); // ok
}

namespace OverloadResolutionTemplate {
  auto f();
  template<typename T> auto f(T);

  int &g(int (*f)()); // expected-note {{not viable: no overload of 'f' matching 'int (*)()'}} expected-note {{candidate}}
  char &g(int (*f)(int)); // expected-note {{not viable: no overload of 'f' matching 'int (*)(int)'}} expected-note {{candidate}}

  int a = g(f); // expected-error {{no matching function}}

  auto f() { return 0; }

  int &b = g(f); // ok (presumably), due to deduction failure forming type of 'f<int>'

  template<typename T> auto f(T) { return 0; }

  int &c = g(f); // expected-error {{ambiguous}}
}

namespace DefaultedMethods {
  struct A {
    auto operator=(const A&) = default; // expected-error {{must return 'DefaultedMethods::A &'}}
    A &operator=(A&&); // expected-note {{previous}}
  };
  auto A::operator=(A&&) = default; // expected-error {{differs from the declaration in the return type}}
}

namespace Constexpr {
  constexpr auto f1(int n) { return n; }
  struct NonLiteral { ~NonLiteral(); } nl; // expected-note {{user-provided destructor}}
  constexpr auto f2(int n) { return nl; } // expected-error {{return type 'Constexpr::NonLiteral' is not a literal type}}
}

// It's not really clear whether these are valid, but this matches g++.
using size_t = decltype(sizeof(0));
auto operator new(size_t n, const char*); // expected-error {{must return type 'void *'}}
auto operator delete(void *, const char*); // expected-error {{must return type 'void'}}

namespace Virtual {
  struct S {
    virtual auto f() { return 0; } // expected-error {{function with deduced return type cannot be virtual}} expected-note {{here}}
  };
  // Allow 'auto' anyway for error recovery.
  struct T : S {
    int f();
  };
  struct U : S {
    auto f(); // expected-error {{different return}}
  };

  // And here's why...
  struct V { virtual auto f(); }; // expected-error {{cannot be virtual}}
  struct W : V { virtual auto f(); }; // expected-error {{cannot be virtual}}
  auto V::f() { return 0; } // in tu1.cpp
  auto W::f() { return 0.0; } // in tu2.cpp
  W w;
  int k1 = w.f();
  int k2 = ((V&)w).f();
}

namespace std_examples {

namespace NoReturn {
  auto f() {}
  void (*p)() = &f;

  auto *g() {} // expected-error {{cannot deduce return type 'auto *' for function with no return statements}}
}

namespace UseBeforeComplete {
  auto n = n; // expected-error {{variable 'n' declared with 'auto' type cannot appear in its own initializer}}
  auto f(); // expected-note {{declared here}}
  void g() { &f; } // expected-error {{function 'f' with deduced return type cannot be used before it is defined}}
  auto sum(int i) {
    if (i == 1)
      return i;
    else
      return sum(i - 1) + i;
  }
}

namespace Redecl {
  auto f();
  auto f() { return 42; }
  auto f(); // expected-note 2{{previous}}
  int f(); // expected-error {{functions that differ only in their return type cannot be overloaded}}
  decltype(auto) f(); // expected-error {{cannot be overloaded}}

  template<typename T> auto g(T t) { return t; } // expected-note {{candidate}}
  template auto g(int);
  template char g(char); // expected-error {{does not refer to a function}}
  template<> auto g(double);

  template<typename T> T g(T t) { return t; } // expected-note {{candidate}}
  template char g(char);
  template auto g(float);

  void h() { return g(42); } // expected-error {{ambiguous}}
}

namespace ExplicitInstantiationDecl {
  template<typename T> auto f(T t) { return t; }
  extern template auto f(int);
  int (*p)(int) = f;
}

}