[Bug libstdc++/106248] [11 Regression] operator>>std::basic_istream at boundary condition behave differently in different opt levels

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106248

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Backported for 12.2