libstdc++: Make std::unique_lock self-move-assignable

LWG 4172 was approved in Hagenberg, February 2025, fixing
std::unique_lock and std::shared_lock to work correctly for
self-move-assignment.

Our std::shared_lock was already doing the right thing (contradicting
the standard) so just add a comment there. Our std::unique_lock needs to
be fixed to do the right thing.

libstdc++-v3/ChangeLog:

	* include/bits/unique_lock.h (unique_lock::operator=): Fix for
	self-move-assignment.
	* include/std/shared_mutex (shared_lock::operator=): Add
	comment.
	* testsuite/30_threads/shared_lock/cons/lwg4172.cc: New test.
	* testsuite/30_threads/unique_lock/cons/lwg4172.cc: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>

2 files changed, 55 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/30_threads/shared_lock/cons/lwg4172.cc b/libstdc++-v3/testsuite/30_threads/shared_lock/cons/lwg4172.cc
new file mode 100644
index 0000000..0a3bf10
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/shared_lock/cons/lwg4172.cc
@@ -0,0 +1,28 @@
+// { dg-do run { target c++14 } }
+
+// LWG 4172. unique_lock self-move-assignment is broken
+
+#include <shared_mutex>
+#include <testsuite_hooks.h>
+
+void
+test_self_move()
+{
+  struct Lockable
+  {
+    bool locked = false;
+    void lock_shared() { locked = true; }
+    void unlock_shared() { locked = false; }
+    bool try_lock_shared() { if (locked) return false; return locked = true; }
+  };
+
+  Lockable x;
+  std::shared_lock<Lockable> l(x);
+  l = std::move(l);
+  VERIFY(x.locked);
+}
+
+int main()
+{
+  test_self_move();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/unique_lock/cons/lwg4172.cc b/libstdc++-v3/testsuite/30_threads/unique_lock/cons/lwg4172.cc
new file mode 100644
index 0000000..37542b5
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/unique_lock/cons/lwg4172.cc
@@ -0,0 +1,27 @@
+// { dg-do run { target c++11 } }
+
+// LWG 4172. unique_lock self-move-assignment is broken
+
+#include <mutex>
+#include <testsuite_hooks.h>
+
+void
+test_self_move()
+{
+  struct Lockable
+  {
+    bool locked = false;
+    void lock() { locked = true; }
+    void unlock() { locked = false; }
+  };
+
+  Lockable x;
+  std::unique_lock<Lockable> l(x);
+  l = std::move(l);
+  VERIFY(x.locked);
+}
+
+int main()
+{
+  test_self_move();
+}