[Issue 24635] Allow opApply with default parameters

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24635

--- Comment #2 from Bolpat <qs.il.paperinik at gmail.com> ---
A use case is a recursive `opApply` that walks a tree-like structure; the
recursive calls calls `opApply` on the child trees.

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