[Bug c++/118230] `std::is_invocable` gives wrong result when the tested function returns `auto`

[harald at gigawatt dot nl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118230

Harald van Dijk <harald at gigawatt dot nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |harald at gigawatt dot nl

--- Comment #1 from Harald van Dijk <harald at gigawatt dot nl> ---
The error is because the static_assert is evaluated while the class is still
being defined. This becomes clearer with a tweaked example:

    #include <utility>
    #include <type_traits>
    #include <functional>
    struct A {
        struct B {
            auto operator()(const auto& i) const /* -> std::size_t */ {
                return std::hash<int>{}(i);
            }
        };
        decltype(B()(123)) x;
        static_assert(
            std::is_invocable_v<const B&, const int&>
        );  // FAILED!!!
    };
    int main() { A{}; }

Here, line 10 results in:

  error: use of 'auto A::B::operator()(const auto:6&) const [with auto:6 =
int]' before deduction of 'auto'

But std::is_invocable_v is tricky, it cannot report this error, it can only
give true or false.