[Issue 21184] Unimplemented methods of interface are not reported as errors during compilation.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21184

--- Comment #7 from alexandru.ermicioi at gmail.com ---
Well, at that point I've failed to think about separate compilation.

Seems like forward declarations are allowed anywhere.

It is still an unpleasant experience when you forget implementing a method and
get mangled method name as an error from linker and not compiler.

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