Spurious type error when joining boxed types with effects

[marzipankaiser]

The following Effekt program (or a similar one with match):

effect Go[T](): Unit

def dropTill[T2](l: (Int => Int / { Go[T2] } at {})): (Int => Int / { Go[T2] } at {}) = 
  if (false) { dropTill[T2](l) } else { l }

it fails with the error message:

{ Go[T2] } is not equal to { Go[T2] }
  l match {
  ^

raised from unifyEffects:

effekt/effekt/shared/src/main/scala/effekt/typer/TypeComparer.scala

Line 83 in cfa951e

if (eff1.toList.toSet != eff2.toList.toSet) error(pp"${eff2} is not equal to ${eff1}", ctx)

called from the join between the result types in the if/match.

The two T2 have a different id (when inspected via the debugger).

[marzipankaiser]

Note that

effect Go[T](): Unit

def dropTill[T2](l: (Int => Int / { Go[T2] } at {})): (Int => Int / { Go[T2] } at {}) = 
  if (false) { box dropTill[T2](l) } else { l }

does compile but we shouldn't have to unbox and box here, right? (It just forwards the boxed function unchanged)