[Issue 13348] std.uni.byGrapheme is impure

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13348

--- Comment #4 from hsteoh at quickfur.ath.cx ---
It's not a problem that byGrapheme is sometimes impure, but it should not be
the one introducing the impurity. I.e., if the incoming range is pure, then
byGrapheme ought to be also pure.

This can be enforced by not attributing byGrapheme directly, but using a pure
nothrow etc. unittest to ensure any impure/throwing/etc. code actually comes
from the incoming range, not from byGrapheme itself.

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