libstdc++: Improve diagnostics for std::packaged_task invocable checks

Moving the static_assert that checks is_invocable_r_v into _Task_state
means it is checked when we instantiate that class template.

Replacing the __create_task_state function with a static member function
_Task_state::_S_create ensures we instantiate _Task_state and trigger
the static_assert immediately, not deep inside the implementation of
std::allocate_shared. This results in shorter diagnostics that don't
show deeply-nested template instantiations before the static_assert
failure.

Placing the static_assert at class scope also helps us to fail earlier
than waiting until when the _Task_state::_M_run virtual function is
instantiated. That also makes the diagnostics shorter and easier to read
(although for C++11 and C++14 modes the class-scope static_assert
doesn't check is_invocable_r, so dangling references aren't detected
until _M_run is instantiated).

libstdc++-v3/ChangeLog:

	* include/std/future (__future_base::_Task_state): Check
	invocable requirement here.
	(__future_base::_Task_state::_S_create): New static member
	function.
	(__future_base::_Task_state::_M_reset): Use _S_create.
	(__create_task_state): Remove.
	(packaged_task): Use _Task_state::_S_create instead of
	__create_task_state.
	* testsuite/30_threads/packaged_task/cons/dangling_ref.cc:
	Adjust dg-error patterns.
	* testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc:
	Likewise.

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

2 files changed, 6 insertions, 7 deletions

diff --git a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc
index 51c6ade..8cc3f78 100644
--- a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc
+++ b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc
@@ -8,6 +8,5 @@ int f();
 std::packaged_task<const int&()> task(f);
 // { dg-error "dangling reference" "" { target { c++14_down } } 0 }
 // { 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 }
+// { dg-error "note: .*std::is_invocable_r" "" { target c++17 } 0 }
diff --git a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc
index 6ba1bb1..b3413c2 100644
--- a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc
+++ b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc
@@ -12,16 +12,16 @@ struct F {
 
 // 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" }
+std::packaged_task<void()> p(f); // { dg-error "here" }
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
+// { dg-error "note: .*std::is_invocable_r_v<void, " "" { target c++17 } 0 }
 
 // Only callable as rvalue
 struct Frv {
   int* operator()() && { return 0; }
 };
-std::packaged_task<int*()> p2(Frv{}); // { dg-error "here" "" { target c++17 } }
+std::packaged_task<int*()> p2(Frv{}); // { dg-error "here" }
+// { dg-error "note: .*std::is_invocable_r_v<int., " "" { target c++17 } 0 }
 
 // Only callable as non-const lvalue
 struct Fnc {