Match type on parameterized trait doesn't preserve input parameter in type variable

[s5bug]

Compiler version

3.1.0, 3.1.1-RC1, 3.1.1-RC2

Minimized code

sealed trait BoundedPair[A, B <: A]

type Second[A, T <: BoundedPair[A, _ <: A]] <: A = T match {
  case BoundedPair[A, b] => b
}

Output

Found:    b
Required: A

where:    b is a type in type Second with bounds <: Playground.BoundedPair[?, ?]#A

Expectation

Second[Animal, BoundedPair[Animal, Cat]] ⇒ Cat

[s5bug]

Note that this is also true with just

sealed trait Single[B]

type Retrieve[A, T <: Single[_ <: A]] <: A = T match {
  case Single[b] => b
}

but I was not sure whether that is the same issue or not.

[OlivierBlanvillain]

Probably related to #11205

[SethTisue]

not sure whether that is the same issue or not

looks like it to me and Dale, this (the Single version) looks like the one to investigate since it's simpler. we're digging into it, but we're still exploring (flailing?) and don't have a theory yet

[SethTisue]

This can be minimized further:

trait Single[B]
type Retrieve[A, T <: Single[A]] <: A = T match
  case Single[b] => b

-- [E007] Type Mismatch Error: -------------------------------------------------
3 |  case Single[b] => b
  |                    ^
  |                    Found:    b
  |                    Required: A

and further:

trait Single[B]
type Retrieve[A] <: A = Single[A] match
  case Single[b] => b

same error.