PR libstdc++/68825
	* include/experimental/bits/shared_ptr.h (__shared_ptr, __weak_ptr,
	experimental::shared_ptr, experimental::weak_ptr): Constrain
	assignment operators.
	* testsuite/experimental/memory/shared_ptr/cons/copy_ctor_neg.cc:
	Change to a compile-only test and change dg-excess-errors to dg-error.

From-SVN: r231516