[Issue 23998] @mustuse should require opCast(T:bool) is checked

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23998

--- Comment #4 from Richard Cattermole <alphaglosined at gmail.com> ---
I'm not saying it has to be opCast, that's just my use case as it works for me
at runtime as-is.

Basically, I need a way to tell the compiler, if any of these specific methods
are not called immediately, then this value has been discarded even if other
methods have been called.

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