Match types parent type not reduced

Question
1

I’m using match types to implement rules for type promotion at the type level.

Here’s a simplified/minimized example (in reality there are more DTypes and we are combining multiple of them):

sealed abstract class DType
final class Int32 extends DType

val int32: Int32 = new Int32

type Promoted[A <: DType] <: DType = A match
  case Int32 => Int32
  case _     => DType

def f[A <: DType](a: A): Promoted[A] = a.asInstanceOf[Promoted[A]]

f(int32) // Int32
f(int32: DType) // Does not reduce: Promoted[DType]
f[DType](int32) // Does not reduce: Promoted[DType]
// We can still do an explicit type ascription: 
f(int32: DType): DType // DType

Scastie

Is there a way way to persuade the compiler to reduce to DType somehow, or is this a fundamental restriction?

2

Promoted[DType] cannot be reduced because DType is not provably disjoint from Int32. Indeed, it clearly is not disjoint.