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.
---
Tested x86_64-linux.
libstdc++-v3/include/std/future | 9 ++++-
.../packaged_task/cons/dangling_ref.cc | 1 +
.../packaged_task/cons/lwg4154_neg.cc | 36 +++++++++++++++++++
3 files changed, 45 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc
diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index 2a855f262a0..b7ab233b85f 100644
--- a/libstdc++-v3/include/std/future
+++ b/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
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 225b65fe6a7..51c6ade91c3 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
@@ -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 }
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
new file mode 100644
index 00000000000..57472a3d798
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc
@@ -0,0 +1,36 @@
+// { 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" }
+
+namespace good
+{
+ struct F {
+ 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 F f;
+ std::packaged_task<void()> p(f);
+}
+
+
--
2.48.1
