[Issue 18751] chunkBy predicate cannot access local variable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18751

--- Comment #2 from hsteoh at quickfur.ath.cx ---
This is related to issue #14909, but the PR only fixes this problem, not that
one, so I'm keeping the bug reports separate for now.

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