Nonunified types in GADT pattern match

Compiler version

3.1.3

Minimized code

enum PingMessage[Response]:
  case Ping(from: String) extends PingMessage[String]

val pongBehavior: [O] => (Unit, PingMessage[O]) => (Unit, O) = [O] =>
  (state: Unit, msg: PingMessage[O]) =>
    msg match
      case PingMessage.Ping(from) => ((), s"Pong from $from")

Scastie: https://scastie.scala-lang.org/omCKwdwSQ2uGyiYSPRbOLQ

Output

Found:    [O] => (Unit, PingMessage[O]) => (Unit, String)
Required: [O] => (Unit, PingMessage[O²]) => (Unit, O²)

where:    O  is a type variable
          O² is a type variable

Expectation

Compiles successfully

[dwijnand]

Again: keep an eye out for variance. If you switch to a covariant Option/Some instead of sticking to something invariant like PingMessage/Ping was, the behaviour is going to be different.

[SethTisue]

Dale and I have been looking at this further.

Our current hypothesis is that the root cause here is that when a polymorphic function literal is desugared, the result type of the apply method is always left blank and it’s up to type inference to fill it in. Desugar.scala line 1734 creates this DefDef:

DefDef(nme.apply, applyTParams :: applyVParams :: Nil, TypeTree(), res)

where the TypeTree() is the missing result type of the apply.

Let’s return to Dale’s minimization. To keep this straight in your head, it’s helpful not to reuse the same type parameter name. So the minimization becomes:

val poly: [O1] => Option[O1] => O1 =    // line 1
  [O2] =>                               // line 2
    (opt: Option[O2]) => ...            // line 3

line 1 is where we say what the expected type of the polymorphic function is, and that expected type includes the information that the body’s result type is O1.

But when lines 2 and 3 are desugared, the desugaring includes def apply(...) = , whereas what we want is def apply(...): O2 = ..., since if we don’t put O2 there, inference puts String there instead, which is wrong.

So we’d need to take the expected type [O1] => Option[O1] => O1, extract the third O1, replace all of the type parameters according to their indices so that O1 becomes O2, and then fill that in.