如何解决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
考虑以下代码(Godbolt):
<div *ngFor="let post of posts">
{{ "category." + post.category | translate}}
</div>
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。
版权声明:本文内容由互联网用户自发贡献,该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务,不拥有所有权,不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容, 请发送邮件至 dio@foxmail.com 举报,一经查实,本站将立刻删除。