[Issue 13936] groupBy must be redone

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13936

--- Comment #46 from bearophile_hugs at eml.cc ---
(In reply to Andrei Alexandrescu from comment #40)
> RefCounted can be made observably pure (by means of casts)

Sounds like a "trusted pure" could be useful.

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