Pattern matcher loses existential information under certain circunstances #6801
Description
While trying to fix some code that the newer pattern matcher doesn't accept, because of invalid F-Bounded Polymorphism constraints, I stumbled upon some situations where the circular type constraint was lost by the pattern matcher.
Here are some examples comparing situations where the information is lost with those where the information is not lost. This was run on the REPL with -Xfull-lubs, just in case.
scala> trait T[A <: T[A]]
defined trait T
scala> def proc1(l1: List[T[A] forSome { type A <: T[A]}]) = for(elem <- l1) yield elem
proc1: (l1: List[T[A] forSome { type A <: T[A] }])List[T[A] forSome { type A <: T[A] }]
scala> def proc2(l1: List[T[A] forSome { type A <: T[A]}], l2: List[T[A] forSome { type A <: T[A]}]) = for ((e1, e2) <- l1 zip l2) yield e2
proc2: (l1: List[T[A] forSome { type A <: T[A] }], l2: List[T[A] forSome { type A <: T[A] }])List[T[_1] forSome { type _1 <: T[_0]; type _0 <: T[A]; type A <: T[A] }]
scala> def proc3(l1: List[T[A] forSome { type A <: T[A]}], l2: List[T[A] forSome { type A <: T[A]}]) = (l1.head, l2.head) match { case (e1, e2) => e2 }
proc3: (l1: List[T[A] forSome { type A <: T[A] }], l2: List[T[A] forSome { type A <: T[A] }])T[A] forSome { type A <: T[A] }
scala> def proc4(l1: List[T[A] forSome { type A <: T[A]}], l2: List[T[A] forSome { type A <: T[A]}]) = l1 zip l2 match { case (e1, e2) :: _ => e2 }
<console>:9: warning: match may not be exhaustive.
It would fail on the following input: Nil
def proc4(l1: List[T[A] forSome { type A <: T[A]}], l2: List[T[A] forSome { type A <: T[A]}]) = l1 zip l2 match { case (e1, e2) :: _ => e2 }
^
proc4: (l1: List[T[A] forSome { type A <: T[A] }], l2: List[T[A] forSome { type A <: T[A] }])T[_1] forSome { type _1 <: T[_0]; type _0 <: T[A]; type A <: T[A] }