[Bug c++/78391] New: g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[lucanus81 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

            Bug ID: 78391
           Summary: g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't
                    warn when class members are used uninitialized.
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lucanus81 at gmail dot com
  Target Milestone: ---

Created attachment 40063
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40063&action=edit
Minimal testcase that shows the (possible) problem.

It seems like g++ (any version) doesn't warn when class members are
uninitialized when no optimization is enabled (and at -O0). For -O1, -O2, -O3
we correctly get a warning (On the contrary, clang, any version, reports a
warning at any optimization level).

struct data { int x=w; int w=10; };

int main() {
    data d;
    return d.x;
}

g++ ub.cpp -Wall -Wextra -std=gnu++1z -O0 
=> no warning

g++ ub.cpp -Wall -Wextra -std=gnu++1z -O1
=> 
ub.cpp: In function 'int main()':
ub.cpp:4:10: warning: 'd.data::w' is used uninitialized in this function
[-Wuninitialized]
     data d;

for -O2, -O3 we get exactly the same warning.