C ++ 20概念：GCC和Clang中带有指针类型成员的复合需求表达式的行为差异

问题描述

考虑以下代码（Godbolt）：

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Clang 10.0.1成功编译了该程序。但是GCC 10.2失败了：

#include <iostream>
//#include <concepts>
#include <type_traits>

// Since the latest clang doesn't have <concepts>,// took this here: https://en.cppreference.com/w/cpp/concepts/same_as
// Using of std::same_as still gives an error in GCC.
namespace detail {
    template< class T,class U >
    concept SameHelper = std::is_same_v<T,U>;
}

template< class T,class U >
concept same_as = detail::SameHelper<T,U> && detail::SameHelper<U,T>;

template<typename T>
concept Hasstr = requires(T a) { { a.str } -> same_as<const char*>; };

struct A {
    const char* str = "A";
};

const char* f(Hasstr auto has_str) {
    return has_str.str;
}

int main() {
    A a;
    std::cout << f(a) << "\n";
    return 0;
}

最有趣的部分是：

<source>: In function 'int main()':
<source>:28:21: error: use of function 'const char* f(auto:11) [with auto:11 = A]' with unsatisfied constraints
   28 |     std::cout << f(a) << "\n";
      |                     ^
<source>:22:13: note: declared here
   22 | const char* f(Hasstr auto has_str) {
      |             ^
<source>:22:13: note: constraints not satisfied
<source>: In instantiation of 'const char* f(auto:11) [with auto:11 = A]':
<source>:28:21:   required from here
<source>:16:9:   required for the satisfaction of 'Hasstr<auto:11>' [with auto:11 = A]
<source>:16:18:   in requirements with 'T a' [with T = A]
<source>:16:38: note: 'a.str' does not satisfy return-type-requirement,because
   16 | concept Hasstr = requires(T a) { { a.str } -> same_as<const char*>; };
      |                                    ~~^~~
<source>:16:36: error: deduced expression type does not satisfy placeholder constraints
   16 | concept Hasstr = requires(T a) { { a.str } -> same_as<const char*>; };
      |                                  ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:16:36: note: constraints not satisfied
<source>:9:13:   required for the satisfaction of 'SameHelper<T,U>' [with T = const char*&; U = const char*]
<source>:13:9:   required for the satisfaction of 'same_as<const char*&,const char*>'
<source>:9:31: note: the expression 'is_same_v<T,U> [with T = const char*&; U = const char*]' evaluated to 'false'
    9 |     concept SameHelper = std::is_same_v<T,U>;
      |                          ~~~~~^~~~~~~~~~~~~~~
Compiler returned: 1

据我了解，在复合要求<source>:9:31: note: the expression 'is_same_v<T,U> [with T = const char*&; U = const char*]' evaluated to 'false' 中，表达式的类型为{ a.str }，而不是const char*&。那么，为什么会这样呢？哪个编译器是正确的？

解决方法

通过我对[expr.prim.req.compound]/1的阅读，GCC正确地发出了错误：

必须满足decltype（（E））的类型约束的立即声明约束（[temp.param]）。

伴随示例：

requires {
  { E1 } -> C;
  { E2 } -> D<A₁,⋯,An>;
};

等同于

requires {
  E1; requires C<decltype((E1))>;
  E2; requires D<decltype((E2)),A₁,An>;
};

decltype((a.str))的确是const char*&，所以我希望这就是传递给same_as的东西。