Scala 2.13 regression: type param affecting overload

Compiler version

3.0.0

Minimized code (ver 1)

final class HookComponentBuilder[Ctx, CtxFn[_]] {
  def asd[A](f: Ctx => A): A = ???
  def asd[A](f: CtxFn[A]): A = ???
}

object HookCtx {
  case class P1[P, H1](props: P, hook1: H1)
}

object HookCtxFn {
  sealed trait P1[P, H1] { type Fn[A] = (P, H1) => A }
}

object Test {
  val b: HookComponentBuilder[HookCtx.P1[String, Int], HookCtxFn.P1[String, Int]#Fn] = ???

  b.asd($ => $.props.length + $.hook1)
  b.asd((props, hook1) => props.length + hook1)
}

Minimized code (ver 2)

Changing HookCtxFn.P1 to a type lambda…

final class HookComponentBuilder[Ctx, CtxFn[_]] {
  def asd[A](f: Ctx => A): A = ???
  def asd[A](f: CtxFn[A]): A = ???
}

object HookCtx {
  case class P1[P, H1](props: P, hook1: H1)
}

object HookCtxFn {
  type P1[P, H1] = [A] =>> (P, H1) => A
}

object Test {
  val b: HookComponentBuilder[HookCtx.P1[String, Int], HookCtxFn.P1[String, Int]] = ???

  b.asd($ => $.props.length + $.hook1)
  b.asd((props, hook1) => props.length + hook1)
}

Output

[error] 18 |  b.asd((props, hook1) => props.length + hook1)
[error]    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |        Wrong number of parameters, expected: 1

Expectation

It should compile. It does with Scala 2.13

Workaround

Inlining HookCtxFn.P1 makes it work.

final class HookComponentBuilder[Ctx, CtxFn[_]] {
  def asd[A](f: Ctx => A): A = ???
  def asd[A](f: CtxFn[A]): A = ???
}

object HookCtx {
  case class P1[P, H1](props: P, hook1: H1)
}

object Test {
  val b: HookComponentBuilder[HookCtx.P1[String, Int], [A] =>> (String, Int) => A] = ???

  b.asd($ => $.props.length + $.hook1)
  b.asd((props, hook1) => props.length + hook1)
}

[odersky]

Thanks for the minimization! I will take another look.

[japgolly]

Also did a quick test to see if all 4 representations are identical. They are:

object Test2 {
  sealed trait Scala2TL { type F[Z] = (String, Int) => Z }
  type Scala3TL = [Z] =>> (String, Int) => Z

  var b1: Builder[[Z] =>> (String, Int) => Z            ] = ???
  var b2: Builder[Scala2TL#F                            ] = ???
  var b3: Builder[Scala3TL                              ] = ???
  var b4: Builder[({ type F[Z] = (String, Int) => Z })#F] = ???

  b1 = b1
  b1 = b2
  b1 = b3
  b1 = b4

  b2 = b1
  b2 = b2
  b2 = b3
  b2 = b4

  b3 = b1
  b3 = b2
  b3 = b3
  b3 = b4

  b4 = b1
  b4 = b2
  b4 = b3
  b4 = b4

  // no errors. good.
}

So this must be just a .dealias needed somewhere in overload resolution?