[Bug c++/105937] New: [12] Regression with maybe-uninitialized

[janisozaur+gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105937

            Bug ID: 105937
           Summary: [12] Regression with maybe-uninitialized
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janisozaur+gcc at gmail dot com
  Target Milestone: ---

Created attachment 53125
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53125&action=edit
Test case extracted from original code

Commit https://gcc.gnu.org/g:6feb628a706e86eb3f303aff388c74bdb29e7381 "Improve
warning suppression for inlined functions [PR98512]. " introduced a regression
when using std::optional.

Ever since that commit, when building optimized code compiler complains:

/home/mijn/workspace/openrct2/src/openrct2/drawing/Drawing.String.cpp: In
function ‘void ttf_process_string_literal(OpenRCT2::IContext*,
std::string_view)’:
/home/mijn/workspace/openrct2/src/openrct2/drawing/Drawing.String.cpp:215:62:
error: ‘*(long unsigned int*)((char*)&ttfRunIndex + offsetof(std::optional<long
unsigned int>,std::optional<long unsigned
int>::<unnamed>.std::_Optional_base<long unsigned int, true,
true>::<unnamed>))’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
  215 |                 auto len = it.GetIndex() - ttfRunIndex.value();
      |                                                              ^


gcc -v:

Using built-in specs.
COLLECT_GCC=/home/mijn/workspace/gcc-install-master/bin/c++
COLLECT_LTO_WRAPPER=/home/mijn/workspace/gcc-install-master/libexec/gcc/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure
--prefix=/home/mijn/workspace/gcc-install-master --enable-languages=c++
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20210702 (experimental) (GCC)



Provided an extracted test case for the issue mentioned.
Original code:
https://github.com/OpenRCT2/OpenRCT2/blob/b0ffa9d28e62b862f68a37788931265cb4528835/src/openrct2/drawing/Drawing.String.cpp#L839
Original bug report for OpenRCT2:
https://github.com/OpenRCT2/OpenRCT2/issues/17371
Test case on godbolt: https://godbolt.org/z/6Y3cn1Ks1

c++ -O2 -c -o foo test.cpp -Werror=maybe-uninitialized