[Issue 9300] Syntax for loop invariants

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=9300

--- Comment #5 from Bolpat <qs.il.paperinik at gmail.com> ---
It’s not entirely clear if an invariant should be checked if the loop is
entered or left via `goto`. My sense is entering should not check the
invariant, but exiting, I truly don’t know. `scope(success)` would check it.

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