DuplicateBind error triggered by same variable name in alternative pattern

[ShaneDelmore]

I believe the issue is in Typer.transform. Here is an example snippet and output:

object Test {
  def g(p:(Int,Int)) = p match {
    case (10,n) | (n,10) => println(n) 
    case _ => println("nope")
  }
}

On compile this error is returned:

-- [E005] Naming Error: test.scala -------------------------------------------------------------------------------------
9 |    case (10,n) | (n,10) => println(n)
  |                   ^
  |                   duplicate pattern variable: `n`

longer explanation available when compiling with `-explain`

[ShaneDelmore]

Tagging @DarkDimius and @liufengyun at @felixmulder request.

[odersky]

There's clearly an error here, but not the one that's flagged. scalac says:

error: illegal variable in pattern alternative

[ShaneDelmore]

@odersky This is actually how I found this bug, I was attempting to improve the error message for "illegal variable in pattern alternative" and found I could not trigger the error message in Dotty as I could in Scala.

[odersky]

@ShaneDelmore You want to give it a try to add the error message? I believe it should be in Typer. The thing to watch out for are Bind nodes that bind a name other than _ inside Alternative nodes.
Maybe add a Mode bit that we are inside an alternative that gets tested in the Bind node?

[ShaneDelmore]

I will take a look at it this weekend and see if I can make any headway on it.

[ShaneDelmore]

I started looking into this bug last weekend and think I should be able to come up with a fix. Besides the error message, is there any reason not to just allow multiple variable bindings if they exist in every alternative with the same name and same type?

[felixmulder]

This sounds like a judgement call for @odersky, the spec doesn't specifically prohibit this. It might make the desugaring inside PatternMatcher harder - unclear.

[odersky]

Duplicate of #3332.