`restrict` uses a predicate function to restrict a type, giving the
restricting a descriptive name in the process.
First, the wrapped type definition is checked (e.g. int) and then the
value is checked with the predicate, so the predicate can already
depend on the value being of the wrapped type.
Change-Id: Ic3edde45a8f34c31bc164414580d0a1aa5a821d5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2312
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
Reviewed-by: tazjin <mail@tazj.in>
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