[Bug c++/126346] New: False positive "may be used uninitialized" with -O3, std::variant and std::string

[gcc at cohi dot at via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126346

            Bug ID: 126346
           Summary: False positive "may be used uninitialized" with -O3,
                    std::variant and std::string
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at cohi dot at
  Target Milestone: ---

Created attachment 65109
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65109&action=edit
PEGTL-repro

Another variant of the "may be used uninitialized" bug in conjunction with -O3
and a std::string inside a std::variant, filing separately as requested here
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107138#c21

I can provoke the false positive with GCC 16.1.0 on macOS in two ways:

- With the small repdroducer from 107138 which needs -O3 and
-fsanitize=undefined.

- With the PEGTL with only -O3 (have not been able to find a smaller repro
yet).

Attached file is the preprocessed PEGTL source that provokes the problem and
the compiler output.