As explained in details in dlang/dmd#11000 and dlang/dmd#11474,
'in' has been for a very long time lowered to simply 'const',
or 'scope const' when v2.092.0's '-preview=in' switch is used.
DMD PR 11474 aims to change this, so 'in' is not (visibly)
lowered, and shows up in any user-visible string.
This includes header generation, error messages, and stringof.
Since this code was testing stringof directly, it is affected
by the change. To support testing of both aforementioned DMD PRs,
the change is not tied to a version,
but uses the (soon-to-be) old way as a fallback.
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