PR libstdc++/79162
	* include/bits/basic_string.h (basic_string::_If_sv): Remove from the
	overload set when the argument is derived from basic_string.
	* testsuite/21_strings/basic_string/cons/char/moveable2_c++17.cc: New
	test.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2_c++17.cc:
	New test.

From-SVN: r253024