[Bug c++/112318] Deprecated move ctor does not trigger -Wdeprecated-declarations when creating a std::optional

["herring at lanl dot gov" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112318

S. Davis Herring <herring at lanl dot gov> changed:

           What    |Removed                     |Added
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                 CC|                            |herring at lanl dot gov

--- Comment #4 from S. Davis Herring <herring at lanl dot gov> ---
The issue is really broader than just "declaration or use is not in -isystem";
since it's standard practice to use -isystem for third-party headers (to avoid
"personal" warnings like -Wparentheses taking effect in code not written to
them), this still fails to issue a warning if `foo` comes from such a header. 
(But the warning works when foo's author checks for it in their own unit
tests...)

It's a very hard problem in general.  A colleague pointed me at Perl's carp,
which attempts to identify whose "fault" a diagnostic is
<https://perldoc.perl.org/Carp#DESCRIPTION>.  The package rule there could even
be applied to C++20 modules, or perhaps to top-level namespaces in a pinch. 
Beyond that, it might be worthwhile to presume that any warning triggered by a
type- or value-dependent expression should be associated (for -Wsystem-headers
purposes) with the point of instantiation, although I'm sure there would be
some false positives from that.  (There needs to be a better-than-#pragma way
to suppress individual deprecation warnings for such cases.)