Declared in <bslstl_stringbuf.h>
template<
typename _CharT2,
typename _Traits2,
typename _Alloc>
basic_istream<_CharT2, _Traits2>&
operator>>(
basic_istream<_CharT2, _Traits2>&,
basic_string<_CharT2, _Traits2, _Alloc>&);
This function is a hidden friend of basic_stringbuf, declared inside its base class basic_streambuf: it can be found only through argument-dependent lookup.