Bug 11178 – Class may implement same interface multiple times with different interface pointers, breaking (a is b) semantics

Consider https://gist.github.com/6847299 . The errant behavior goes away if you remove the I2 inheritance from C2, indicating that C2 implements I2 _twice_ with different interface pointers. This leads to hilariously hard-to-find errors, since one generally assumes that casting two objects to the same interface makes them is-comparable if they're the same object. The second I2 in C2 should either be made illegal or silently ignored.

Pasting in test case in comments just incase gist.github removes the pastebin. =) module testcase; import std.stdio; interface I1 { } interface I2 : I1 { } class C1 : I2 { void foo() { writefln("These should REALLY not be the same:"); writefln(": %s", cast(void*) cast(I2) cast(I1) this); writefln(": %s", cast(void*) cast(I2) this); writefln("for comparison"); Object obj = cast(Object) this; writefln(": %s", cast(void*) cast(I2) cast(I1) obj); writefln(": %s", cast(void*) cast(I2) obj); } } class C2 : C1, I2 { } void main() { (new C2).foo(); }

Of course, I meant to say "These should REALLY be the same" there. Typo, sorry.

*** This issue has been marked as a duplicate of issue 4979 ***