Match Type Fails on Generic Type

[godenji]

Compiler version

3.1.0

Minimized code

type Id = Int | Long | UUID

trait MappedTo[+T <: Id] extends Any:
  def value: T

type GetType[T <: MappedTo[_]] = T match
  case MappedTo[Int] => Int
  case MappedTo[Long] => Long
  case MappedTo[UUID] => UUID

class Extract[T <: MappedTo[_]](val map: T => GetType[T])

given extract[T <: MappedTo[_]]: Extract[T] = new Extract[T](_.value) // ERROR

Output

-- Error:
given extract[T <: MappedTo[_]]: Extract[T] = Extract[T](_.value)
   |                                                     ^^^^^^^
   |            Found:    _$1.T
   |            Required: GetType[T²]
   |
   |            where:    T  is a type in trait MappedTo with bounds <: Id
   |                      T² is a type in given instance extract with bounds <: MappedTo[?]
   |
   |
   |            Note: a match type could not be fully reduced:
   |
   |              trying to reduce  GetType[T]
   |              failed since selector  T
   |              does not match  case MappedTo[Int] => Int
   |              and cannot be shown to be disjoint from it either.
   |              Therefore, reduction cannot advance to the remaining cases
   |
   |                case MappedTo[Long] => Long
   |                case MappedTo[java.util.UUID] => java.util.UUID

Expectation

That runtime concrete T resolves GetType[T] match to underlying Int | Long | UUID.

Perhaps this a feature request (for lazy match types), and not a bug report, but regardless, for scenarios like this where only a type param is available (i.e. no value), it is impossible to replace removed Abstract Type Projections in Scala 3, which permanently breaks some Scala 2 libraries.

[odersky]

This is definitely a feature request.

[godenji]

This is definitely a feature request.

Unless abstract type projections are reinstated, this feature would seem like quite a useful addition to the language.

Sitting on two broken Scala 2 libraries here with no solution in sight...