This is the last remaining piece of P0935R0. This adds a default
constructor to each of the streambuf and stream types in <sstream> so
that default construction does not use the 'explicit' constructor that
has a single, defaulted argument.
P0935R0 Eradicating unnecessarily explicit default constructors
* config/abi/pre/gnu.ver: Tighten existing patterns and export new
default constructor symbols.
* include/std/sstream (basic_stringbuf, basic_istringstream)
(basic_ostringstream, basic_stringstream): Remove default arguments
from explicit constructors taking ios_base::openmode and add separate
non-explicit default constructors.
* testsuite/27_io/basic_istringstream/cons/default.cc: New.
* testsuite/27_io/basic_ostringstream/cons/default.cc: New.
* testsuite/27_io/basic_stringstream/cons/default.cc: New.
* testsuite/27_io/basic_stringbuf/cons/char/default.cc: New.
* testsuite/27_io/basic_stringbuf/cons/wchar_t/default.cc: New.
From-SVN: r262474
