[Bug libstdc++/83427] New: [7/8 Regression] [C++17] weak result types and adaptable functions don't support noexcept functions

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83427

            Bug ID: 83427
           Summary: [7/8 Regression] [C++17] weak result types and
                    adaptable functions don't support noexcept functions
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

This compiles in C++11 and C++14 mode, but fails for C++17 due to noexcept in
the function type:

#include <functional>
int f() noexcept { return 0; }
std::reference_wrapper<decltype(f)> r(f);
decltype(r)::result_type i;
auto b = std::bind(f);
decltype(b)::result_type j;

ne.cc:4:14: error: ‘result_type’ in ‘class std::reference_wrapper<int()
noexcept>’ does not name a type
 decltype(r)::result_type i;
              ^~~~~~~~~~~
ne.cc:6:14: error: ‘result_type’ in ‘struct std::_Bind<int (*())() noexcept>’
does not name a type
 decltype(b)::result_type j;
              ^~~~~~~~~~~