[Issue 21321] Class with unimplemented interface method compiles, links, then segfaults, if inherited through abstract base class

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21321

--- Comment #6 from Adam D. Ruppe <destructionator at gmail.com> ---
And I hit it AGAIN today because in the interface, I wrote the wrong return
type and used the right type in the class. The compiler SHOULD have said
"original one not implemented" and drew my attention to it. Instead, it just
randomly crashed with a null vtable entry.

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