libstdc++: Add static_assert to std::packaged_task::packaged_task(F&&)

LWG 4154 (approved in Wrocław, November 2024) fixed the Mandates:
precondition for std::packaged_task::packaged_task(F&&) to match what
the implementation actually requires. We already gave a diagnostic in
the right cases as required by the issue resolution, so strictly
speaking we don't need to do anything. But the current diagnostic comes
from inside the implementation of std::__invoke_r and could be more
user-friendly.

For C++17 (when std::is_invocable_r_v is available) add a static_assert
to the constructor, so the error is clear:

.../include/c++/15.0.1/future: In instantiation of 'std::packaged_task<_Res(_ArgTypes ...)>::packaged_task(_Fn&&) [with _Fn = const F&; <template-parameter-2-2> = void; _Res = void; _ArgTypes = {}]':
lwg4154_neg.cc:15:31:   required from here
   15 | std::packaged_task<void()> p(f); // { dg-error "here" "" { target c++17 } }
      |                               ^
.../include/c++/15.0.1/future:1575:25: error: static assertion failed
 1575 |           static_assert(is_invocable_r_v<_Res, decay_t<_Fn>&, _ArgTypes...>);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Also add a test to confirm we get a diagnostic as the standard requires.

libstdc++-v3/ChangeLog:

	* include/std/future (packaged_task::packaged_task(F&&)): Add
	static_assert.
	* testsuite/30_threads/packaged_task/cons/dangling_ref.cc: Add
	dg-error for new static assertion.
	* testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc: New
	test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

libstdc++-v3/include/std/future

@@ -1567,7 +1567,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	packaged_task(_Fn&& __fn)
 	: _M_state(
 	    __create_task_state<_Res(_ArgTypes...)>(std::forward<_Fn>(__fn)))
-	{ }
+	{
+#ifdef __cpp_lib_is_invocable // C++ >= 17
+	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+	  // 4154. The Mandates for std::packaged_task's constructor
+	  // from a callable entity should consider decaying
+	  static_assert(is_invocable_r_v<_Res, decay_t<_Fn>&, _ArgTypes...>);
+#endif
+	}
 
 #if __cplusplus < 201703L
       // _GLIBCXX_RESOLVE_LIB_DEFECTS

libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc

@@ -10,3 +10,4 @@ std::packaged_task<const int&()> task(f);
 // { dg-error "reference to temporary" "" { target { c++14_down } } 0 }
 // { dg-error "no matching function" "" { target c++17 } 0 }
 // { dg-error "enable_if" "" { target c++17 } 0 }
+// { dg-error "static assertion failed" "" { target c++17 } 0 }

libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc

@@ -0,0 +1,38 @@
+// { dg-do compile { target c++11 } }
+
+// LWG 4154. The Mandates for std::packaged_task's constructor from
+// a callable entity should consider decaying
+
+#include <future>
+
+struct F {
+  void operator()() & = delete;
+  void operator()() const & { }
+};
+
+// Mandates: is_invocable_r_v<R, decay_t<F>&, ArgTypes...> is true.
+const F f;
+std::packaged_task<void()> p(f); // { dg-error "here" "" { target c++17 } }
+// { dg-error "static assertion failed" "" { target c++17 } 0 }
+// { dg-error "invoke_r" "" { target *-*-* } 0 }
+// { dg-prune-output "enable_if<false" }
+
+// Only callable as rvalue
+struct Frv {
+  int* operator()() && { return 0; }
+};
+std::packaged_task<int*()> p2(Frv{}); // { dg-error "here" "" { target c++17 } }
+
+// Only callable as non-const lvalue
+struct Fnc {
+  void operator()() const & = delete;
+  void operator()() & { }
+};
+
+// In C++11/14/17/20 std::packaged_task::packaged_task<F>(F&&) incorrectly
+// required that the callable passed to the constructor can be invoked.
+// If the type of the parameter is const F& we might not be able to invoke
+// the parameter, but that's OK because we store and invoke a non-const F.
+// So this should work without errors:
+const Fnc fnc;
+std::packaged_task<void()> p3(fnc);