signature of implementation of a generic method in trait is not checked

[kory33]

The following code should not compile, as it allows unsound constructs:

object Test {

  trait A { def polymorphic[x]: Int }
  val a = new A { val polymorphic = Unit }

}

(compiles in both 0.10.0-RC1 and 0.11.0-bin-20181113-bf5ba46-NIGHTLY.)

The code fails as expected when trait is replaced with abstract class.

[odersky]

Fixed by #5622