[Issue 13936] groupBy must be redone

[via Digitalmars-d-bugs]

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

--- Comment #39 from hsteoh at quickfur.ath.cx ---
I've run into some roadblocks: RefCounted is @system which makes groupBy
@system. This can be mostly swept under the carpet by making the GroupBy ctor
@trusted, but there's a further problem: RefCounted is also (inherently)
impure, which makes this design unusable in pure code. :-(

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