[Bug libstdc++/87538] New: Incorrect noexcept specifier for not_fn

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87538

            Bug ID: 87538
           Summary: Incorrect noexcept specifier for not_fn
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Found by Edgar Rokjān (https://stackoverflow.com/q/52673235/2069064). The
noexcept specifier for _Not_fn currently only checks the noexcept-ness of
applying the ! operator on the return type, not of the actual call:

noexcept(noexcept(_S_not<__inv_res_t<_Fn _QUALS, _Args...>>()))

As a result, this fails:

#include <functional>

struct X { 
    int operator()(int);
};

static_assert(!noexcept(
    std::not_fn(X{})(2)));

While not_fn isn't actually specified to have any kind of noexcept-specifier,
this one seems actively wrong - either it should propagate through the type's
actual call function (so the above assert shouldn't trigger) or it should never
be noexcept (likewise).