PR libstdc++/81381
	* include/bits/sstream.tcc (basic_stringbuf::overflow)
	(basic_stringbuf::basic_stringbuf(const __string_type&, ios::mode))
	(basic_stringbuf::str()): Construct new strings with an allocator.
	* testsuite/27_io/basic_stringbuf/cons/81381.cc: New.

From-SVN: r250101