Missing override checks

[smarter]

Missing override keyword

class A

class X {
  def f: A = ???
}
class Y extends X {
  def f: A = ???
}

scalac output:

override.scala:7: error: overriding method f in class X of type => A;
 method f needs `override' modifier
  def f: A = ???
      ^
one error found

Dotty does not detect that error.

Invalid overriding

class A
class B

class X {
  def f: A = ???
}
class Y extends X {
  override def f: B = ???
}

scalac output:

override.scala:8: error: overriding method f in class X of type => A;
 method f has incompatible type
  override def f: B = ???
               ^
one error found

Dotty does not detect that error either.

CC @odersky @DarkDimius @namin

[j2rodrig]

Another bit of information on this issue: The examples above don't generate calls to the checkOverride method in dotc.typer.RefChecks. However, this code does:

class A
class B
class X {
  def f: A = ???
}
class Y extends X {
  override def f: B = ???
}
class Z extends Y {}   // triggers a call to checkOverride for symbol f

This code generates 2 calls to checkOverride with the same parameters:

class A
class B
class X {
  def f: A = ???
}
class Y extends X {
  override def f: B = ???
}
class Z extends Y {}    // triggers a call to checkOverride for symbol f
class Z1 extends Y {}   // triggers another call to checkOverride for symbol f

[smarter]

Fixed by #228.