Is gcc wrongly evaluating std::declval in this concept definition?

Question

In this concept definition:

#include <utility>

template<class Func, class Ret, class... Args>
concept Invokable = requires(Func f) {
    { f(std::declval<Args>()...) } -> Ret;
};

when instantiated like this:

static_assert(Invokable<decltype([](int){}), void, int>);

gcc-9.0.1 (trunk) dumps (well, the Standard Library implementation to be precise):

$ g++ -O2 -std=c++2a -fconcepts -Wall -Wextra -Werror -c tu1.cpp
error: static assertion failed: declval() must not be used!
2204 |       static_assert(__declval_protector<_Tp>::__stop,
     |                                               ^~~~~~

demo: https://godbolt.org/z/D0ygU4

Is it wrong to reject this code? If not, what did I do wrong? If yes, where should one report this bug?

---

Notes

This is accepted

template<auto f, class... Args>
constexpr auto size_of_return_type = sizeof(f(std::declval<Args>()...));

when instantiated like so:

static_assert(sizeof(int) == size_of_return_type<[](int){ return 0; }, int>);

Demo: https://godbolt.org/z/gYGk8U

Latest C++2a draft states:

[expr.prim.req]/2 A requires-expression is a prvalue of type bool whose value is described below. Expressions appearing within a requirement-body are unevaluated operands.

Answer 1

Is it wrong to reject this code?

Yes, concepts are never evaluated, as evidenced in the quote you're citing. This is gcc bug 68781 and gcc bug 82171.

请注意，没有理由declval在概念内使用。这更简单：

template<class Func, class Ret, class... Args>
concept InvokableR = requires(Func&& f, Args&&... args) {
    { f(std::forward<Args>(args)...) } -> Ret;
};

declval之所以存在，是因为您需要某种类型的表达式，而不能仅仅T()因为需要默认的构造函数而编写代码。概念使您具备一流的语言功能。仍然需要forward。