[Bug libstdc++/70564] New: Problem with std::experimental::not_fn

[rcc.dark at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70564

            Bug ID: 70564
           Summary: Problem with std::experimental::not_fn
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rcc.dark at gmail dot com
  Target Milestone: ---

Hi all. The following code does not compile under MinGW-64 GCC 5.2.0 / Linux-64
GCC 5.3.1:

#include <experimental/functional>
#include <array>

int main( )
{
   auto lambda = [](auto i) {
      return i == 5;
   };

   std::array<int, 100> arr;
   std::find_if(arr.begin( ), arr.end( ), std::experimental::not_fn(lambda));
}

Inside std::find_if, an object of the _Iter_Pred class is created:

      _Iter_pred(_Predicate __pred)
        : _M_pred(__pred)
      { }

which calls _Not_fn<PRED>(_Not_fn<PRED>&) since __pred is an lvalue. However,
between these _Not_fn constructors:

      template<typename _Fn2>
        explicit
        _Not_fn(_Fn2&& __fn) : _M_fn(std::forward<_Fn2>(__fn)) { }

        _Not_fn(const _Not_fn& __fn) = default;
        _Not_fn(_Not_fn&& __fn) = default;

the first one wins and triggers an error.