[PATCH 3/3] libstdc++: Fix std::not_fn perfect forwarding [PR111327]

[Patrick Palka <ppalka@redhat.com>]

The previous patch fixed perfect forwarding for std::bind_front.
This patch fixes the same issue for std::not_fn.

Tested on x86_64-pc-linux-gnu, does this look OK for trunk and
perhaps 13?

	PR libstdc++/111327

libstdc++-v3/ChangeLog:

	* include/std/functional (_GLIBCXX_NOT_FN_CALL_OP): Also define
	a deleted fallback operator() overload.  Constrain both the
	main and deleted overloads accordingly.
	* testsuite/20_util/function_objects/not_fn/111327.cc: New test.
---
 libstdc++-v3/include/std/functional           | 10 +++++--
 .../20_util/function_objects/not_fn/111327.cc | 29 +++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc

diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index c50b9e4d365..9551e38dfdb 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -1061,7 +1061,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // forwarding _M_fn and the function arguments with the same qualifiers,
       // and deducing the return type and exception-specification.
 #define _GLIBCXX_NOT_FN_CALL_OP( _QUALS )				\
-      template<typename... _Args>					\
+      template<typename... _Args,					\
+	       typename = enable_if_t<__is_invocable<_Fn _QUALS, _Args...>::value>> \
 	_GLIBCXX20_CONSTEXPR						\
 	decltype(_S_not<__inv_res_t<_Fn _QUALS, _Args...>>())		\
 	operator()(_Args&&... __args) _QUALS				\
@@ -1070,7 +1071,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	{								\
 	  return !std::__invoke(std::forward< _Fn _QUALS >(_M_fn),	\
 				std::forward<_Args>(__args)...);	\
-	}
+	}								\
+									\
+      template<typename... _Args,					\
+	       typename = enable_if_t<!__is_invocable<_Fn _QUALS, _Args...>::value>> \
+	void operator()(_Args&&... __args) _QUALS = delete;
+
       _GLIBCXX_NOT_FN_CALL_OP( & )
       _GLIBCXX_NOT_FN_CALL_OP( const & )
       _GLIBCXX_NOT_FN_CALL_OP( && )
diff --git a/libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc b/libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc
new file mode 100644
index 00000000000..93e00ee8057
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc
@@ -0,0 +1,29 @@
+// PR libstdc++/111327 - std::bind_front (and std::not_fn) doesn't perfectly
+// forward according to value category of the call wrapper object
+// { dg-do compile { target c++17 } }
+
+#include <functional>
+#include <utility>
+
+struct F {
+  void operator()(...) & = delete;
+  bool operator()(...) const &;
+};
+
+struct G {
+  void operator()(...) && = delete;
+  bool operator()(...) const &&;
+};
+
+int main() {
+  auto f = std::not_fn(F{});
+  f(); // { dg-error "deleted" }
+  std::move(f)();
+  std::as_const(f)();
+  std::move(std::as_const(f))();
+
+  auto g = std::not_fn(G{});
+  g(); // { dg-error "deleted" }
+  std::move(g)(); // { dg-error "deleted" }
+  std::move(std::as_const(g))();
+}
-- 
2.42.0.158.g94e83dcf5b