Match expression aliases types without considering type bounds

[fhackett]

minimized code

class X[A,B]

abstract class A[T]

class B[F <: X[_,_]] extends A[F]

def fn1[T](a : A[T]) = a match {
  case aa : B[f] => fn2(aa)
}

def fn2[F <: X[_,_]](b : B[F]) = ???

This blows up on the call to fn2 saying that type parameter T does not conform to upper bound X[_,_]. While this is true, we just matched a to aa which is of type B[f], where f should by construction have the correct bounds. Somehow Dotty is considering f === T, ignoring changes in bounds due to the match.

expectation

This code compiles without the error described above.

Note: this might be related to #6697, since that looks similar but on the type level

[liufengyun]

@AleksanderBG It seems related to GADT. Feel free to reassign if it's not related.

[DmytroMitin]

Not reproducible in 3.2.2 https://scastie.scala-lang.org/DmytroMitin/eECHNH2IRdCKRUNPU4x0Vg