Type check mismatch

Compiler version

3.0.0

Minimized code

case class G[A, X[_]](x: X[A])

enum Boxes[A]:
  case IntBox(a: Int)
  case StringBox(a: String)
import Boxes._

def f(i: Int): G[Int, Boxes] =
  val box = IntBox(i)
  return G(box)

Output

Found:    (box : Boxes[Nothing])
Required: Boxes[Int]
  return G(box)

Expectation

Behave the same as the in-line version (which does work)

...
def g(i: Int): G[Int, Boxes] =
  return G(IntBox(i))

[dwijnand]

This is, as I understand it, fundamentally down to the fact that the type system has subtyping.

[Adam-Vandervorst]

The typer is working with information it has during type inference. It is not taking into account potential future uses of the value.

Maybe taking into account future uses is something useful to do, especially in trivial cases like this. Going along that, the error messaging could be improved here: Found: (box : Boxes[Nothing]) implies this type was given by the user. In more difficult cases where the “taking into account future uses” fails, a clearer error message could read:

Could not find a type for
  val box = IntBox(i)
that satisfies required type(s): 
Boxes[Int]
  return G(box)

This may significantly improve the comfort, while avoiding breaking the equivalence stated above as much as possible.