Re: std::jthread::operator=(std::jthread&&) calls std::terminate if *this has an associated running thread.

[Paul Scharnofske <asynts@gmail.com>]

>I think this would work:
> 
>   jthread& operator=(jthread&& __x) noexcept
>   {
>     std::jthread(std::move(__x)).swap(*this);
>     return *this;
>   }

That looks a lot better than what I did, it's also consistent with other
places like std::stop_token::operator=(std::stop_token&&).

I updated my patch and also created a test that checks the post
conditions described in the standard.

---
 libstdc++-v3/include/std/thread               |  6 +++++-
 .../testsuite/30_threads/jthread/jthread.cc   | 20 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index 887ee579962..080036e2609 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 746ff437c1d..b8ba62f6df2 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();
 }
-- 
2.29.2