libstdc++: Assigning to a joinable std::jthread calls std::terminate

Move assigning to a std::jthread that represents a thread of execution
needs to send a stop request and join that running thread. Otherwise the
std::thread data member will terminate in its assignment operator.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/thread (jthread::operator=(jthread&&)): Transfer
	any existing state to a temporary that will request a stop and
	then join.
	* testsuite/30_threads/jthread/jthread.cc: Test move assignment.

2 files changed, 25 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index 887ee57..080036e 100644
--- a/libstdc++-v3/include/std/thread
+++ b/libstdc++-v3/include/std/thread
@@ -456,7 +456,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     operator=(const jthread&) = delete;
 
     jthread&
-    operator=(jthread&&) noexcept = default;
+    operator=(jthread&& __other) noexcept
+    {
+      std::jthread(std::move(__other)).swap(*this);
+      return *this;
+    }
 
     void
     swap(jthread& __other) noexcept
diff --git a/libstdc++-v3/testsuite/30_threads/jthread/jthread.cc b/libstdc++-v3/testsuite/30_threads/jthread/jthread.cc
index 746ff43..b8ba62f 100644
--- a/libstdc++-v3/testsuite/30_threads/jthread/jthread.cc
+++ b/libstdc++-v3/testsuite/30_threads/jthread/jthread.cc
@@ -187,6 +187,25 @@ void test_detach()
   VERIFY(t1FinallyInterrupted.load());
 }
 
+//------------------------------------------------------
+
+void test_move_assignment()
+{
+    std::jthread thread1([]{});
+    std::jthread thread2([]{});
+
+    const auto id2 = thread2.get_id();
+    const auto ssource2 = thread2.get_stop_source();
+
+    thread1 = std::move(thread2);
+
+    VERIFY(thread1.get_id() == id2);
+    VERIFY(thread2.get_id() == std::jthread::id());
+
+    VERIFY(thread1.get_stop_source() == ssource2);
+    VERIFY(!thread2.get_stop_source().stop_possible());
+}
+
 int main()
 {
   std::set_terminate([](){
@@ -197,4 +216,5 @@ int main()
   test_stop_token();
   test_join();
   test_detach();
+  test_move_assignment();
 }