[Bug libstdc++/125371] Missing fixit hint for forward declaration w/ std::istringstream and missing include

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125371

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't think we want to change how the front end handles types that are
declared but not defined, this was just a QoI issue in the library headers. So
fixed for GCC 17.