Why you want to run `cl.exe /vmg`

output.txt

$ cl.exe test.cpp && test.exe
10 4
$ g++ test.cpp -o test && ./test
10 10
$ cl.exe /vmg test.cpp && test.exe
10 10

test.cpp

#include <cstdio>

class ForwardDeclared;

class Defined {
  void foo();
};

struct ContainerA {
 void (ForwardDeclared::*ptr)();
};

struct ContainerB {
 void (Defined::*ptr)();
};

int main() {
  printf("%lx %lx\n", sizeof(ContainerA), sizeof(ContainerB));
  return 0;
}