https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54043
--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> --- The result is supposed to be a null-terminated string, so we could do what glibc's printf does for null pointers and print "(nil)" but we'd have to widen the string to the stream's char_type. Alternatively we could just print '0'. Either way we probably need to define the function for C++11 or compile the src/c++11/ostream-inst.cc file as C++17 so that the explicit instantiation definition includes the new member function.
