[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 #8 from Walter Bright <bugzilla at digitalmars.com> ---
Template mixins are meant to be included multiple times. That's why the scope
is part of the mangled name.

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