Valid pattern match on this.type rejected as neither super nor subtype

[abgruszecki]

enum Expr[A] {
  case BoolLit(b: Boolean) extends Expr[Boolean]
  def eval: A = {
    def go[AA](self: this.type & Expr[AA]): A = self match {
      case BoolLit(b) => b
    }
    go(this)
  }
}

Error message:

  |Pattern type Expr.BoolLit[A] is neither a subtype nor a supertype of selector type Expr'[A'](Expr'.this) & Expr'[AA]
  |
  |where:    A     is a type variable
  |          A'    is a type in class Expr
  |          Expr  is a object
  |          Expr' is a class

Motivation for why someone would want to do this: it could allow "indirectly" constraining type parameters of enclosing class.

[abgruszecki]

Was fixed in #6398.