That indeed looks ambiguous to me. I expected the typical "if A is well formed then do A else if ..." chain in the standard wording, but it's not used here.


On 30 October 2022 03:29:41 CET, Eric Schmidt via Std-Discussion <std-discussion@lists.isocpp.org> wrote:
In [func.require], we have the definition of INVOKE(f, t1, t2, ..., tN), which has a number of cases. The first two are

(t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T and is_­base_­of_­v<T, remove_­reference_­t<decltype(t1)>> is true;


(t1.get().*f)(t2, ..., tN) when f is a pointer to a member function of a class T and remove_­cvref_­t<decltype(t1)> is a specialization of reference_­wrapper;

There is no mention of which of these is to be preferred if the conditions hold in both cases.

So, given

using refwrap = std::reference_wrapper<int>;

is std::is_invocable_v<void (refwrap::*)(), refwrap> true? If the first case takes precedence, then yes. If the second case, then no.

A quick check on Godbolt shows that GCC and Clang evaluate the previous expression to false, while MSVC evaluates it to true.

A similar ambiguity exists for the pointer-to-data-member case.

(I had originally written the following example, which yields a compiler error on GCC and Clang, but is accepted by MSVC. But then I realized that it violates the general ban on creating pointers to library functions.)

void f(std::reference_wrapper<int> x)
std::invoke(&std::reference_wrapper<int>::get, x);
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