[Issue 19850] double nested joiner .back falsely thinks the range is empty

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19850

--- Comment #1 from FeepingCreature <default_357-line at yahoo.de> ---
Shorter repro: [[[0]]].map!joiner.joiner.back;

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