Unable to write a PolyFunction lambda with a "match type" return type

[weihsiu]

Minimized code

  type Upgrade[T] = T match
    case Int => Double
    case Char => String
    case Boolean => Boolean

  // compiles
  val upgrade: [t] => t => Upgrade[t] = new PolyFunction:
    def apply[T](x: T): Upgrade[T] = x match
      case x: Int => x.toDouble
      case x: Char => x.toString
      case x: Boolean => !x
  
  // does not compile
  val upgrade2: [t] => t => Upgrade[t] = [t] => (x: t) => x match
      case x: Int => x.toDouble
      case x: Char => x.toString
      case x: Boolean => !x

Output

[error] -- [E007] Type Mismatch Error: /.../Tuples.scala:42:27 e / compileIncremental 0s
[error] 42 |      case x: Boolean => !x
[error]    |                           ^
[error]    |       Found:    Object with PolyFunction {...}
[error]    |       Required: PolyFunction{apply: [t](x$1: t): tlp.Tuples.Upgrade[t]}

Expectation

it should compile.

btw, polyfunction lambda with "regular" return types works just fine.

[prolativ]

This might be somehow related to #6904

[bishabosha]

the issue appears to be that the right hand side of your polymorphic function lambda can't infer Upgrade[T]. adding an explicit ascription works:

val upgrade2 = [t] => (x: t) =>
  (x match
    case x: Int => x.toDouble
    case x: Char => x.toString
    case x: Boolean => !x): Upgrade[t]