LWG3788: `jthread::operator=(jthread&&)` postconditions are unimplementable under self-assignment

[ldionne]

Link: https://wg21.link/LWG3788

[H-G-Hristov]

This appears to have been implemented in LLVM18: 695138c

llvm-project/libcxx/include/__thread/jthread.h

Lines 72 to 84 in ef98e24

	_LIBCPP_HIDE_FROM_ABI jthread& operator=(jthread&& __other) noexcept {
	if (this != &__other) {
	if (joinable()) {
	request_stop();
	join();
	}
	__stop_source_ = std::move(__other.__stop_source_);
	__thread_ = std::move(__other.__thread_);
	}
	return *this;
	}

llvm-project/libcxx/test/std/thread/thread.jthread/assign.move.pass.cpp

Lines 80 to 113 in ef98e24

	// then assigns the state of x to *this
	{
	std::jthread j1 = support::make_test_jthread([] {});
	std::jthread j2 = support::make_test_jthread([] {});
	auto id2 = j2.get_id();
	auto ssource2 = j2.get_stop_source();
	j1 = std::move(j2);
	assert(j1.get_id() == id2);
	assert(j1.get_stop_source() == ssource2);
	}
	// sets x to a default constructed state
	{
	std::jthread j1 = support::make_test_jthread([] {});
	std::jthread j2 = support::make_test_jthread([] {});
	j1 = std::move(j2);
	assert(j2.get_id() == std::jthread::id());
	assert(!j2.get_stop_source().stop_possible());
	}
	// joinable is false
	{
	std::jthread j1;
	std::jthread j2 = support::make_test_jthread([] {});
	auto j2Id = j2.get_id();
	j1 = std::move(j2);
	assert(j1.get_id() == j2Id);
	}