incorrect type checking in pattern matching

[liufengyun]

Following code passes type checking in dotty:

object Test {
  def foo(x: Option[Int]) = x match {
    case Some(_: Double) => true
    case None => true
  }
}

In scalac it generates following error message:

error: scrutinee is incompatible with pattern type;
 found   : Double
 required: Int
    case Some(_: Double) => true
                 ^
one error found

[DarkDimius]

I propose to make it both more general and simpler by decoupling the transformation from typer and doing it after pattern matcher.

I propose to intercept sc.isInstanceOf[Sel]: and, depending on the erased type of sc and Sel, either:

• report the error\warning if test is always failing;
• replace the call be constant if the result is statically known;
• leave as is.

Here is the table for transformation:

Sel\sc	Trait	Class	final Class
Trait	happens	happens	known statically
Class	happens	false if classes are unrelated	known statically
Final class	false if classes are unrelated	false if classes are unrelated	known statically

[liufengyun]

@felixmulder There is an interesting bug with scalac related to this issue, which you may want to add to the test set:

case class A(v: Int)
class B(v: Int) extends A(v)

object Test {
  val a = new B(1)
  a match { case A(_) => 1 }
}

The code above looks perfectly fine, but scalac generates an error:

error: constructor cannot be instantiated to expected type;
 found   : A
 required: B
  a match { case A(_) => 1 }
                 ^
one error found

However, if we annotate a with val a:A = new B(1), everything is fine.

SI-9795.

[felixmulder]

This issue was resolved in #1275