[Issue 19532] chunkBy assert error involving reference input ranges.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #4 from Jon Degenhardt <jrdemail2000-dlang at yahoo.com> ---
A bit more explanation - I encountered problems with chunkBy when writing an
merge and aggregation of sorted files. Basically, an external merge sort,
except also combining equivalent key entries and aggregating values associated
with the keys. This is essentially a merge-chunkby-map/fold operation, except
with a custom merge operation.

The examples in this report are cases I found while trying to identify a
simpler sample case. These samples still don't show all the behaviors I
encountered, but they should be enough to motivate the issues.

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