[Bug c++/78141] New: Missing uninitialized warning about struct member after change another struct definition (without optimization)

[vmario1986 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78141

            Bug ID: 78141
           Summary: Missing uninitialized warning about struct member
                    after change another struct definition (without
                    optimization)
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vmario1986 at gmail dot com
  Target Milestone: ---

Created attachment 39917
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39917&action=edit
Example code demonstrating the issue

I compile this simple program:

#include <cstdio>
#include <iostream>

using namespace std;

struct Foo
{
    int a;
    int b;
};

struct Bar
{
    //Bar() = default;
    int d;
};

int main()
{
    Foo foo;
    Bar bar;

    printf("%d %d\n", foo.a, foo.b);

    return 0;
}

and I get those warnings:

$ g++ -std=c++11 -Wall -Wextra -Wpedantic foo.cpp -o foo
foo.cpp: In function ‘int main()’:
foo.cpp:21:9: warning: unused variable ‘bar’ [-Wunused-variable]
     Bar bar;
         ^
foo.cpp:23:11: warning: ‘foo.Foo::b’ is used uninitialized in this function
[-Wuninitialized]
     printf("%d %d\n", foo.a, foo.b);
           ^
foo.cpp:23:11: warning: ‘foo.Foo::a’ is used uninitialized in this function
[-Wuninitialized]

Of course, this is what we expect. But when I uncomment the Bar default ctor,
there is a problem - all warnings disappear.

With -O1, -O2, -O3 and -Os my code gives warnings in both cases.

I have posted this issue on the Stack Overflow and users found this issue on
GCC 6.1.0 too.