[Issue 21171] Undefined identifier when alias inside static if.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21171

--- Comment #2 from Iain Buclaw <ibuclaw at gdcproject.org> ---
I thought I recognized the problem.  It's likely a duplicate of issue 17883,
which I encountered a few years back.

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