https://issues.dlang.org/show_bug.cgi?id=23388

          Issue ID: 23388
           Summary: ImportC: redeclaration of struct in different
                    translation unit doesn’t check compatibility
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: accepts-invalid, ImportC
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: dave287091@gmail.com

// a.c
struct Foo {
    int x;
};
// b.c
struct Foo {
    int x, y;
};
//d.d
import a;
import b;

static assert(a.Foo.sizeof != b.Foo.sizeof); // Fails

The two structs are assumed to be the same type, size etc. and merged into one instead of iterating over their fields and ensuring all match (in other words, checking for compatibility before assuming they are the same).

--