[Issue 20012] export inside mixin doesn't seem to work

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20012

--- Comment #16 from anonymous4 <dfj1esp02 at sneakemail.com> ---
Callbacks are more common than reflection-based dll exports. For some reason
C++ doesn't allow nested extern "C" functions and they all get mangled.

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