Structure with same name, different definitions: segmentation fault with -O2

I've encountered a segmentation fault in a C++ program when two C++ files compiled together each include a different definition of a structure (with the same name).

According to this question, I understand that structure definitions are restricted to the translation unit (the file and its inclusions).

However, I get a crash when enabling -O1 or more at compile time. The following minimal code reproduced the segfault.

The code is in 3 short C++ files and 2 headers:

// td_collision1.cc
#include <iostream>
#include <vector>
#include <cstdlib>
#include "td1.h"

struct Data
{
  long a;
  double m1;
  double m2;
};

void sz1(void) {
    std::cout << "Size of in collision1: " << sizeof(struct Data) << std::endl;
}

void collision1(void) {
    struct Data tmp;
    std::vector<struct Data> foo;
    for (int i=0; i<10; i++) {
        tmp.a = 1;
        tmp.m1 = 0;
        tmp.m2 = 0;
        foo.push_back(tmp);
    }
}
// td1.h
#include <iostream>

void collision1(void);
void sz1(void);

// td_collision2.cc
#include <iostream>
#include <vector>
#include <cstdlib>
#include "td2.h"

struct Data {
  long a;
  double m1; // note that there is one member less here
};

void sz2(void) {
    std::cout << "Size of in collision2: " << sizeof(struct Data) << std::endl;
}

void collision2(void) {
    struct Data tmp2;
    std::vector<struct Data> bar;
    for (int i=0; i<100; i++) {
        tmp2.a = 1;
        tmp2.m1 = 0;
        bar.push_back(tmp2); // errors occur here
    }
}
// td2.h
#include <iostream>

void collision2(void);
void sz2(void);

// td_main.cc
#include <iostream>
#include <cstdlib>
#include "td1.h"
#include "td2.h"

int main(void) {
    sz1();
    sz2();
    collision2();
}

This code compiled with GCC 6.3 with -O0 flag runs fine and without error under valgrind. However, running it with -O1 or O2 leads to the following output:

Size of in collision1: 24
Size of in collision2: 16
==326== Invalid write of size 8
==326==    at 0x400F6C: construct<Data, const Data&> (new_allocator.h:120)
==326==    by 0x400F6C: construct<Data, const Data&> (alloc_traits.h:455)
==326==    by 0x400F6C: push_back (stl_vector.h:918)
==326==    by 0x400F6C: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326==  Address 0x5aba1f0 is 0 bytes after a block of size 96 alloc'd
==326==    at 0x4C2E1FC: operator new(unsigned long) (vg_replace_malloc.c:334)
==326==    by 0x400DE9: allocate (new_allocator.h:104)
==326==    by 0x400DE9: allocate (alloc_traits.h:416)
==326==    by 0x400DE9: _M_allocate (stl_vector.h:170)
==326==    by 0x400DE9: void std::vector<Data, std::allocator<Data> >::_M_emplace_back_aux<Data const&>(Data const&) (vector.tcc:412)
==326==    by 0x400F7E: push_back (stl_vector.h:924)
==326==    by 0x400F7E: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326== 
==326== Invalid write of size 8
==326==    at 0x400F69: construct<Data, const Data&> (new_allocator.h:120)
==326==    by 0x400F69: construct<Data, const Data&> (alloc_traits.h:455)
==326==    by 0x400F69: push_back (stl_vector.h:918)
==326==    by 0x400F69: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326==  Address 0x5aba1f8 is 8 bytes after a block of size 96 alloc'd
==326==    at 0x4C2E1FC: operator new(unsigned long) (vg_replace_malloc.c:334)
==326==    by 0x400DE9: allocate (new_allocator.h:104)
==326==    by 0x400DE9: allocate (alloc_traits.h:416)
==326==    by 0x400DE9: _M_allocate (stl_vector.h:170)
==326==    by 0x400DE9: void std::vector<Data, std::allocator<Data> >::_M_emplace_back_aux<Data const&>(Data const&) (vector.tcc:412)
==326==    by 0x400F7E: push_back (stl_vector.h:924)
==326==    by 0x400F7E: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326== 
==326== 
==326== HEAP SUMMARY:
==326==     in use at exit: 0 bytes in 0 blocks
==326==   total heap usage: 5 allocs, 5 frees, 73,896 bytes allocated
==326== 
==326== All heap blocks were freed -- no leaks are possible
==326== 
==326== For counts of detected and suppressed errors, rerun with: -v
==326== ERROR SUMMARY: 191 errors from 2 contexts (suppressed: 0 from 0)

the push_back() function fails when the libc reallocates std::vector<struct Data> bar. (in my case, its size is 4 items initially, and the vector is further resized afterwards when calling push_back() in the loop.) When struct Data in td_collision1.cc has the same size as in td_collision2.cc, the program doesn't crash.

Therefore, there seem to be a collision between these two structure definitions. Indeed, if I rename one structure, the bug obviously vanishes. But, as mentioned above, I thought that this could not happen. What did I misunderstood? Also, if I get rid of function collision1(), the segfault vanishes (struct Data in collision1 is probably ditched by the compiler because unused)

My understanding was that there exist a clear separation between these two CC files and no "crosstalk" should be possible if the structures are not present in the header.

Edit: add missing td2.h

The answer you linked is for C language, and C is not C++.

In C++ (quote from en.cppreference, see Danh's answer for the standard), the rule is as follow:

There can be more than one definition in a program, as long as each definition appears in a different translation unit, of each of the following: class type [...], as long as all of the following is true:

• each definition consists of the same sequence of tokens (typically, appears in the same header file)

• [...]

If all these requirements are satisfied, the program behaves as if there is only one definition in the entire program. Otherwise, the behavior is undefined.

Your two definitions clearly violates the first condition, so the behavior is undefined.