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An extractor pattern `X(p)` should type check for any `X.unapply`/`X.unapplySeq`
that returns an `Option[_]` -- previously we were confused about the case where
it was an `Option[(T1, ... , Tn)]`. In this case, the expected type for the
pattern `p` is simply `(T1, ... , Tn)`.
While I was at it, tried to clean up unapplyTypeList and friends (by replacing them by extractorFormalTypes).
From the spec:
8.1.8 ExtractorPatterns
An extractor pattern x(p1, ..., pn) where n ≥ 0 is of the same syntactic form as a constructor pattern.
However, instead of a case class, the stable identifier x denotes an object which has a member method named unapply or unapplySeq that matches the pattern.
An unapply method in an object x matches the pattern x(p1, ..., pn) if it takes exactly one argument and one of the following applies:
n = 0 and unapply’s result type is Boolean.
n = 1 and unapply’s result type is Option[T], for some type T.
the (only) argument pattern p1 is typed in turn with expected type T
n > 1 and unapply’s result type is Option[(T1, ..., Tn)], for some types T1, ..., Tn.
the argument patterns p1, ..., pn are typed in turn with expected types T1, ..., Tn
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