[Issue 19101] New: Miscompilation on extern(C++) overloads with D types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19101

          Issue ID: 19101
           Summary: Miscompilation on extern(C++) overloads with D types
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: johanengelen at weka.io

The following code is miscompiled:


```
// File a.d
module a;

private struct A {
    int a;
}

extern(C++) int foo(T)(T t) {
    return T.sizeof;
}

int return4() {
    A a;
    return foo(a);
}
```


```
// File b.d
module b;

import a;

private struct A {
    int[100] a;
}

void main() {
    import std.stdio;
    A a;
    writeln(foo(a), " =? ", A.sizeof);
    writeln(return4());
}
```

Compile and run:
> dmd a.d b.d
> ./a
4 =? 400
4

The problem is that in module a, `foo!(a.A)(a.A)` is mangled the same as module
b's `foo!(b.A)(b.A)`, because the extern(C++) function name mangler does not
use module name prefix for D types. That is, the mangler is mangling
`foo!(A)(A)` instead of `foo!(a.A)(a.A)` and `foo!(b.A)(b.A)`. The two function
symbols are merged by the linker (instead of erroring on multiple definition),
because the symbols come from templates and merging is required behavior.  -->
only one of the two _different_ definitions survive, and hence miscompilation
results.
The fix: use the full D type for mangling.

--