[Issue 24646] `is` test broken for invalid bools

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24646

--- Comment #1 from Bolpat <qs.il.paperinik at gmail.com> ---
Masking the `bool` for the `is` test is wrong. A struct with padding bits has
its padding bits compared by `is` as well. For all intents and purposes, a
`bool` is a struct around a `ubyte` with the invariant that its value is only
ever 0 or 1.

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