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 [英] C++20 Concepts: Difference in the behavior of the compound requirement expression with a pointer-type member in GCC and Clang
问题描述
考虑以下代码( Godbolt ):
#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;
}
Clang 10.0.1成功编译了该程序.但是GCC 10.2失败了:
Clang 10.0.1 successfully compiles this program. But GCC 10.2 fails:
<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
最有趣的部分是:
<source>:9:31: note: the expression 'is_same_v<T, U> [with T = const char*&; U = const char*]' evaluated to 'false'
据我了解,在复合要求中, {a.str}
表达式的类型为 const char *&
而不是 const char *
.那么,为什么会这样呢?哪个编译器是正确的?
As I understand it, in the compound requirement { a.str }
expression has type const char*&
instead of const char*
. So, why is this happening? Which compiler is correct?
推荐答案
By my reading of [expr.prim.req.compound]/1, GCC is correct to emit an error:
必须满足decltype((E))的类型约束的立即声明约束([temp.param]).
The immediately-declared constraint ([temp.param]) of the type-constraint for decltype((E)) shall be satisfied.
附带示例:
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))
确实是 const char *&
,所以我希望这就是传递给 same_as
.
decltype((a.str))
is indeed const char*&
, so I would expect that this is what's passed to same_as
.
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