[Issue 20079] [2.087.0] Forward declaration of functions defined in mixin templates don't compile

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20079

--- Comment #1 from Ethan Watson <gooberman at gmail.com> ---
Created attachment 1757
  --> https://issues.dlang.org/attachment.cgi?id=1757&action=edit
Code example

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