[Issue 17927] [scope] scope input return value can be escaped

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17927

--- Comment #1 from Martin Nowak <code at dawg.eu> ---
My bad, there are two bugs.

All of the above opSlice methods should fail to compile, but on the mutable and
cost method do, the inout silently compiles, even though it escapes a field.

All of the methods should and do compile with return scope (even the inout
one).
The compiler doesn't infer scope for their returned slice and thus allows
escaping that.

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