From type/Metamodel/DefiniteHOW

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method accepts_type($definite_type, $checkee)

Performs a type-check of $checkee against a definite type. This will check if $checkee is of its base type and matches its definiteness, returning True if they match or False otherwise. This metamethod can get called when the definite type is on the right-hand side of a smartmatch, for instance.