Add support for ReadonlyArray to Array.isArray #22942
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Fixes #17002
This makes the
Array.isArray
test assert that the argument isReadonlyArray
instead ofArray
. This actually works well as the compiler preserves theArray
-ness if the input could have been a fullArray
instead of just aReadonlyArray
; as demonstrated by the test.This is arguably a breaking change if the argument actually is
any
and the user was expecting to get a mutable array from the output; but blindly mutating someany
just because it passesArray.isArray
sounds like a terrible idea so I think it's a good break.