[Bug c++/103375] New: [12 regression] -Werror=uninitialized false positive on unittest-cpp

[slyfox at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103375

            Bug ID: 103375
           Summary: [12 regression] -Werror=uninitialized false positive
                    on unittest-cpp
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
  Target Milestone: ---

Initially build failure noticed on unittest-cpp-2.0.0. I suspect it's a week
old regression.

Here is the minimal extracted example:

$ cat TestTestRunner.cpp
    struct TestReporter {};
    struct TestRunner
    {
        explicit TestRunner(TestReporter& reporter);
          // /*somewhere in .cpp:*/ : reporter_ (reporter) {}
        TestReporter & reporter_; // only store reference

        void RunAndReport();
    };
    struct FixtureBase
    {
        FixtureBase() : runner(reporter) {}
        TestRunner runner;
        TestReporter reporter;
    };
$ g++-12.0.0 -O2 -Wall -Werror -c TestTestRunner.cpp
    TestTestRunner.cpp: In constructor 'FixtureBase::FixtureBase()':
    TestTestRunner.cpp:12:32: error: member 'FixtureBase::reporter' is used
uninitialized [-Werror=uninitialized]
       12 |         FixtureBase() : runner(reporter) {}
          |                                ^~~~~~~~
    cc1plus: all warnings being treated as errors


I think progaram's intent is to only store an object's reference at
construction time. And use it only in the rest of methods after construction is
done.

$ LANG=C g++-12.0.0 -v
Using built-in specs.
COLLECT_GCC=/nix/store/fxn6ca5x1yb1xj8rskkvw5zajrnj8y79-gcc-12.0.0/bin/g++
COLLECT_LTO_WRAPPER=/nix/store/fxn6ca5x1yb1xj8rskkvw5zajrnj8y79-gcc-12.0.0/libexec/gcc/x86_64-unknown-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with:
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20211121 (experimental) (GCC)