[Issue 18386] mixin ... isn't a template error when used in new scope

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18386

--- Comment #2 from Nick Treleaven <nick at geany.org> ---
Also you should use a string mixin, `mixin(b!a);` - not a template mixin.

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