Match type reduction inhibition

[Bersier]

Compiler version

3.5.0-RC1

Minimized code

inline def foo[I <: Int](i: I): ZeroOne[I] = i match
  case _: 0 => 0
  case _ => 1

// Wrapping the return value and type somehow inhibits proper match type reduction
inline def bar[I <: Int](i: I): W[ZeroOne[I]] = i match
  case _: 0 => W(0)
  case _ => W(1)

class W[S <: Int & Singleton](s: S)

type ZeroOne[I <: Int] <: Int = I match
  case 0 => 0
  case _ => 1

Compiler Output

Found:    W[(0 : Int)]
Required: W[ZeroOne[I]]

Expectation

No compile-error.

[Bersier]

The same thing happens with type members:

inline def bar[I <: Int](i: I): W{ type S = ZeroOne[I] } = i match
  case _: 0 => WImpl(0)
  case _ => WImpl(1)

trait W:
  type S <: Int
class WImpl[T <: Int](s: T) extends W:
  type S = T

type ZeroOne[I <: Int] <: Int = I match
  case 0 => 0
  case _ => 1

[dwijnand]

These two are similar to #21425, in that we don't infer the constraints that would make this work. The original foo works because its cases match one-to-one with the type cases of ZeroOne.