libstdc++: Fix self-move for std::weak_ptr [PR108118]

I think an alternative fix would be something like:

  _M_ptr = std::exchange(rhs._M_ptr, nullptr);
  _M_refcount = std::move(rhs._M_refcount);

The standard's move-and-swap implementation generates smaller code at
all levels except -O0 and -Og, so it seems simplest to just do what the
standard says.

libstdc++-v3/ChangeLog:

	PR libstdc++/108118
	* include/bits/shared_ptr_base.h (weak_ptr::operator=):
	Implement as move-and-swap exactly as specified in the standard.
	* testsuite/20_util/weak_ptr/cons/self_move.cc: New test.

(cherry picked from commit 92eb0adc14)

libstdc++-v3/include/bits/shared_ptr_base.h

@@ -1700,9 +1700,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __weak_ptr&
       operator=(__weak_ptr&& __r) noexcept
       {
-	_M_ptr = __r._M_ptr;
-	_M_refcount = std::move(__r._M_refcount);
-	__r._M_ptr = nullptr;
+	__weak_ptr(std::move(__r)).swap(*this);
 	return *this;
       }
 

libstdc++-v3/testsuite/20_util/weak_ptr/cons/self_move.cc

@@ -0,0 +1,19 @@
+// { dg-do run { target c++11 } }
+
+#include <memory>
+#include <testsuite_hooks.h>
+
+void
+test_self_move()
+{
+  std::shared_ptr<int> sp(new int(66));
+  std::weak_ptr<int> wp(sp);
+  wp = std::move(wp); // PR libstdc++/108118
+  std::shared_ptr<int> sp2(wp);
+  VERIFY(sp2 == sp);
+}
+
+int main()
+{
+  test_self_move();
+}