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Re: Ambiguous specification of INVOKE?

From: Lénárd Szolnoki <cpp_at_[hidden]>
Date: Sun, 30 Oct 2022 10:10:03 +0100

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_at_[hidden]> 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);
>Std-Discussion mailing list

Received on 2022-10-30 09:10:23