[Bug libstdc++/66998] New: not_fn invocation is not SFINAE friendly

["tomaszkam at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

            Bug ID: 66998
           Summary: not_fn invocation is not SFINAE friendly
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomaszkam at gmail dot com
  Target Milestone: ---

According to the
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4529.html#func.not_fn
the following invocations not_fn(f)(args...) and !INVOKE(f, args...) (in case
when f is function object it can be simplified to !f(args)). However the in
current implementation they behavior differs in case when they are used in
SFINAE context.

For example:
#include <experimental/functional>

template<typename F, typename Arg>
auto foo1(F f, Arg arg) -> decltype(!f(arg)) { return !f(arg); }

template<typename F, typename Arg>
auto foo1(F f, Arg arg) -> decltype(!f()) { return !f(); }

template<typename F, typename Arg>
auto foo2(F f, Arg arg) -> decltype(not_fn(f)(arg)) { return not_fn(f)(arg); }

template<typename F, typename Arg>
auto foo2(F f, Arg arg) -> decltype(not_fn(f)()) { return not_fn(f)(); }

struct negator
{
    bool operator()(int) const { return false; }
    void operator()() const {}
};

int main()
{
    foo1(negator{}, 1);
//    foo2(negator{}, 1)); //Shall be equivalent to line above. Produces an
compilation error cause by instatiation error during return type deduction.
};

The problem is caused by the use of return type deduction (decltype(auto))
instead of decltype(expr) in the implementation of not_fn. Notice that such use
does not break the function precondition placed in requires clause.