Duplicate pattern variable on generated variables

[Blaisorblade]

Pattern matches have to be linear in type variables. But apparently Dotty demands the same in generated type variables as well, at least if they come from a single type variable.

Sample code:

sealed trait Root[T]
case object C1 extends Root[Int]
case object C2 extends Root[String]
//case class C3[X, Y]() extends Root[X|Y|(X => X)]
case class C3[X, Y]() extends Root[(X => X)|(Y => Y)|(X => Y)]
case class C4[X, Y]() extends Root[(X => X)|(Y => Y)|(X => Y)]

object TestGADT {
  //type Foo // abstract

  def f[A <: Seq[_], B, Foo >: A => B](v: Root[Foo], u: Root[Foo]) = (v, u) match {
    //case C1 =>
    //case C2 =>
    case (C3(), C3()) =>
  }
  //f(C3[Int, Int]())
  //implicitly[Int <:< Int|(Int => Int)]
  //implicitly[(Int => Int) <:< String|(Int => Int)]//($conforms[Int => Int])
  //f[Int, Int, Int|(Int => Int)](C3[Int, Int]())
  //f(C3[Int, Int]())
  f(C3[Seq[_], Long](), C4[Seq[_], Long]())
}

Error (with -explain, on commit 4149833):

-- [E005] Naming Error: /Users/pgiarrusso/git/dotty/tests/IterableTest.scala:157:12
157 |    case (C3(), C3()) =>
    |            ^
    |            duplicate pattern variable: `_$6`

Explanation
===========
For each case bound variable names have to be unique. In:

case (C3(), C3()) =>

`_$6` is not unique. Rename one of the bound variables!

one error found

[Blaisorblade]

Assigned to @liufengyun, please reassign if my guess was bad.