GADT patmat doesn't narrow class type parameters

scala> enum Expr[T] {
           case IExpr(value: Int) extends Expr[Int]
           case BExpr(value: Boolean) extends Expr[Boolean]

           def join(other: Expr[T]): Expr[T] = (this, other) match {
             case (IExpr(i1), IExpr(i2)) => IExpr(i1 + i2)
             case (BExpr(b1), BExpr(b2)) => BExpr(b1 & b2)
           }
         }
6 |      case (IExpr(i1), IExpr(i2)) => IExpr(i1 + i2)
  |                                     ^^^^^^^^^^^^^^
  |                                     found:    Expr[Int]
  |                                     required: Expr[T]
  |
7 |      case (BExpr(b1), BExpr(b2)) => BExpr(b1 & b2)
  |                                     ^^^^^^^^^^^^^^
  |                                     found:    Expr[Boolean]
  |                                     required: Expr[T]
  |

This is similar to an old issue where class/trait parameters aren’t narrowed.

[LPTK]

So how do you propose to distinguish between the object Expr and the trait Expr in error messages?

Why would the compiler’s error messages not follow the Scala fact that a value and a type sharing the same name are not ambiguous? (They live in different namespaces.) At the very least it should not have to introduce a difference between the name of a class and its companion object.

I propose:

   |                                    where: value Expr is an object
   |                                            type Expr is a trait

[abgruszecki]

Well, this was actually implemented in #11222, we just forgot about this particular issue. Closing now.