libstdc++/66998 Make std::experimental::not_fn SFINAE-friendly.

diff --git a/libstdc++-v3/include/experimental/functional b/libstdc++-v3/include/experimental/functional
index c6b9800..9db5fef 100644
--- a/libstdc++-v3/include/experimental/functional
+++ b/libstdc++-v3/include/experimental/functional
@@ -376,8 +376,11 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// Generalized negator.
   template<typename _Fn>
-    struct _Not_fn
+    class _Not_fn
     {
+      _Fn _M_fn;
+
+    public:
       template<typename _Fn2>
 	explicit
 	_Not_fn(_Fn2&& __fn) : _M_fn(std::forward<_Fn2>(__fn)) { }
@@ -389,34 +392,43 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       ~_Not_fn() = default;
 
       template<typename... _Args>
-	decltype(auto)
+	auto
 	operator()(_Args&&... __args)
+	noexcept(noexcept(!_M_fn(std::forward<_Args>(__args)...)))
+	-> decltype(!_M_fn(std::forward<_Args>(__args)...))
 	{ return !_M_fn(std::forward<_Args>(__args)...); }
 
       template<typename... _Args>
-	decltype(auto)
+	auto
 	operator()(_Args&&... __args) const
+	noexcept(noexcept(!_M_fn(std::forward<_Args>(__args)...)))
+	-> decltype(!_M_fn(std::forward<_Args>(__args)...))
 	{ return !_M_fn(std::forward<_Args>(__args)...); }
 
       template<typename... _Args>
-	decltype(auto)
+	auto
 	operator()(_Args&&... __args) volatile
+	noexcept(noexcept(!_M_fn(std::forward<_Args>(__args)...)))
+	-> decltype(!_M_fn(std::forward<_Args>(__args)...))
 	{ return !_M_fn(std::forward<_Args>(__args)...); }
 
       template<typename... _Args>
-	decltype(auto)
+	auto
 	operator()(_Args&&... __args) const volatile
+	noexcept(noexcept(!_M_fn(std::forward<_Args>(__args)...)))
+	-> decltype(!_M_fn(std::forward<_Args>(__args)...))
 	{ return !_M_fn(std::forward<_Args>(__args)...); }
-
-    private:
-      _Fn _M_fn;
     };
 
   /// [func.not_fn] Function template not_fn
-  template <class _Fn>
+  template<typename _Fn>
     inline auto
     not_fn(_Fn&& __fn)
-    { return _Not_fn<std::decay_t<_Fn>>{std::forward<_Fn>(__fn)}; }
+    noexcept(std::is_nothrow_constructible<std::decay_t<_Fn>, _Fn&&>::value)
+    {
+      using __maybe_type = _Maybe_wrap_member_pointer<std::decay_t<_Fn>>;
+      return _Not_fn<typename __maybe_type::type>{std::forward<_Fn>(__fn)};
+    }
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace fundamentals_v2
diff --git a/libstdc++-v3/testsuite/experimental/functional/not_fn.cc b/libstdc++-v3/testsuite/experimental/functional/not_fn.cc
index 8285ec4..4c137e8 100644
--- a/libstdc++-v3/testsuite/experimental/functional/not_fn.cc
+++ b/libstdc++-v3/testsuite/experimental/functional/not_fn.cc
@@ -20,6 +20,8 @@ 
 #include <experimental/functional>
 #include <testsuite_hooks.h>
 
+using std::experimental::not_fn;
+
 int func(int, char) { return 0; }
 
 struct F
@@ -33,8 +35,6 @@  struct F
 void
 test01()
 {
-  using std::experimental::not_fn;
-
   auto f1 = not_fn(func);
   VERIFY( f1(1, '2') == true );
 
@@ -50,8 +50,36 @@  test01()
   VERIFY( f5(1) == false );
 }
 
+template<typename F, typename Arg>
+auto foo(F f, Arg arg) -> decltype(not_fn(f)(arg)) { return not_fn(f)(arg); }
+
+template<typename F, typename Arg>
+auto foo(F f, Arg arg) -> decltype(not_fn(f)()) { return not_fn(f)(); }
+
+struct negator
+{
+    bool operator()(int) const { return false; }
+    void operator()() const {}
+};
+
+void 
+test02()
+{
+  foo(negator{}, 1); // PR libstdc++/66998
+}
+
+void
+test03()
+{
+  struct X { bool b; };
+  X x{ false };
+  VERIFY( not_fn(&X::b)(x) );
+}
+
 int
 main()
 {
   test01();
+  test02();
+  test03();
 }