Parameter untupling can be prevented with indirection

[coreywoodfield]

Compiler version

3.3.4

Minimized code

doesn't work:

trait Foo {
  type T <: Tuple
  type Mapped = Tuple.Map[T, Option]

  def map[A](f: Mapped => A): Unit = ???
}
class Bar extends Foo {
  override type T = (Int, String)
  // doesn't compile
  map { (int, string) =>
    ()
  }
}

works:

trait Foo {
  type T <: Tuple
  
  def map[A](f: Tuple.Map[T, Option] => A): Unit = ???
}
class Bar extends Foo {
  override type T = (Int, String)
  // compiles fine
  map { (int, string) =>
    ()
  }
}

Output

-- [E086] Syntax Error: .../Test.scala:39:22 
39 |  map { (int, string) =>
   |        ^
   |        Wrong number of parameters, expected: 1
40 |    ()
   |----------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   | The function literal
   |
   |     (int, string) =>
   |   {
   |     ()
   |   }
   |
   | has 2 parameters. But the expected type
   |
   |     Bar.this.Map => Unit
   |
   | requires a function with 1 parameters.
    ----------------------------------------------------------------------------
one error found

Expectation

I expect parameter untupling to occur regardless of type indirection, as long as the type can be fully resolved where the parameter untupling should occur. Instead, parameter untupling does not always occur. I think the alias and the match type are both playing a role here: simply having two aliases that refer to a concrete tuple type (type T = (Int, String), type X = T) does not prevent parameter untupling

[WojciechMazur]

This issue was fixed in Scala Next releases, to be exact it now works correctly since release 3.5.0.
Bisect points it was fixed by #19871
Scala 3.3.4 contains all safe-to-backport changes introduced until Scala 3.5.1. It means that we're not able to fix this issue in the LTS without breaking binary/source compatibility in the LTS line, introducing new regressions, or it depended on other changes that failed this criteria.

[Gedochao]

Seems like it's a wontfix for the LTS.
Closing this, nothing to do here.

[som-snytt]

Duplicates #19821