[Bug libstdc++/87614] User related warnings are hidden in system headers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87614

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, that warning seems useful. The user has caused a conversion that alters
the value. The fact it happens deep inside libstdc++ headers is only because
the library templates preserve the types all the way down the call stack.