[Bug c++/72826] New: Poor diagnostic for uninitialized structure field

["zackw at panix dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72826

            Bug ID: 72826
           Summary: Poor diagnostic for uninitialized structure field
           Product: gcc
           Version: 6.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zackw at panix dot com
  Target Milestone: ---

This cut-down test program...

extern "C" int puts(const char *);

struct countdownTimer {

    countdownTimer() : paused{ true } {}

    void start() noexcept {
        if (started) return;
        started = true;
        puts("timer started");
    }

private:
    char dummy[32];
    bool paused;
    bool started;
};

int main() {
    countdownTimer timer;
    timer.start();
}

... produces these diagnostics when compiled with `-O2 -Wall [-std=c++11]`:

test2.cc: In function ‘int main()’:
test2.cc:8:13: warning: ‘*((void*)& timer +33)’ is used uninitialized in this
function [-Wuninitialized]
         if (started) return;
             ^~~~~~~
test2.cc:20:20: note: ‘*((void*)& timer +33)’ was declared here
     countdownTimer timer;
                    ^~~~~

The behavior is the same in g++ 5 and 6, except that you need -std=c++11 with
g++ 5.

`*((void*)& timer +33)` is internal representation-ese that should not be
dumped into a diagnostic. It should somehow manage to produce `timer.started`
instead.