Imprecise type inferred with polymorphic type alias

[fhackett]

Trying to compile this code yields an error I can't make sense of:

package towers.computes

import quoted._

sealed abstract class Computes[T]

object Computes {

  opaque type Opaque[T] = Int

  implicit class ComputesApplication1[T : Type](fn : Computes[Opaque[T]]) {
    def apply[A](arg1 : Computes[A]) : Computes[T] = ???
  }

  def let[V, T : Type](value : Computes[V], body : Computes[Opaque[T]]) : Computes[T] = body(value)
}

The error is:

[error] -- [E007] Type Mismatch Error: /home/finn/programming/polyparse/src/main/scala/Computes.scala:15:92
[error] 15 |  def let[V, T : Type](value : Computes[V], body : Computes[Opaque[T]]) : Computes[T] = body(value)
[error]    |                                                                                        ^^^^^^^^^^^
[error]    |                             Found:    towers.computes.Computes[Nothing]
[error]    |                             Required: towers.computes.Computes[T]

For some reason, the .apply method on ComputesApplication1 is determined to yield Computes[Nothing] instead of Computes[T] as the signature dictates.

While trying to minimise my example, here are some variations I tried:

• Removing the : Type implicits prevents the bug from occurring.
• Replacing Type with a trait declared trait SomeImplicit[T] does not cause the same issue
• Removing only the : Type in the let method changes nothing, while it should fail because the ComputesApplication1 would then be missing an implicit it needs.
• Removing the Computes[...] and just leaving Opaque[...] causes what's left to just compile normally

[nicolasstucki]

It can be minimized as

sealed abstract class Computes[T]

object Computes {

  type Type[Y]

  type Foo[T] = Int

  class ComputesApplication1[T: Type](fn : Computes[Foo[T]]) {
    def apply() : Computes[T] = ???
  }

  def let[X : Type](body : Computes[Foo[X]]) : Computes[X] = {
    implicit val x: Type[Nothing] = ??? // Should not be needed, looked for as ComputesApplication1[Nothing] requires it
    new ComputesApplication1(body).apply()
  }
}

Therefore it has nothing to do with opaque types, the issue is more general.
For ComputesApplication1 we are inferring the bounds Nothing <:< T <:< X and then choose the lower bound as it is the most precise one.