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Ambiguous specification of INVOKE?

From: Eric Schmidt <eric41293_at_[hidden]>
Date: Sat, 29 Oct 2022 19:29:41 -0700
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

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);

Received on 2022-10-30 02:29:52