missing override check

[olhotak]

abstract class C[T] { def head: T }
case class D[T](head: Int) extends C[T]

Notice the type of head in D is (incorrectly) Int instead of T.

This compiles in dotty but not in scalac.

[olhotak]

The types of the two heads are:
ExprType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,<empty>)),D)),C$$T))
TypeRef(TermRef(ThisType(TypeRef(NoPrefix,<root>)),scala),Int)

OverridingPairs calls matches on them and gets false. It therefore considers them not to override, so their return types are never checked for consistency.

[olhotak]

Here is an even simpler example of the missing check, not involving a case class:

abstract class A { def foo: Int }
class B extends A { val foo: String = "foo" }

Scalac says:

overriding.scala:2: error: overriding method foo in class A of type => Int;
 value foo has incompatible type
class B extends A { val foo: String = "foo" }
                        ^
one error found

Dotc does not report any error.

Interestingly, if I change the val in class B to a def, then dotc does report the error.

[olhotak]

In the last example, the two non-matching types are:

ExprType(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,<root>)),scala),Int))
TypeRef(TermRef(ThisType(TypeRef(NoPrefix,scala)),Predef),String)

[olhotak]

Here is another example where dotty still fails to find an error:

class A {
  def m: String = "foo"
}

class B extends A {
  override val m: Int = 42
}

class C extends A {
  override def m: Int = 42
}

Scalac finds errors in both class B and class C. Dotty finds an error in class C, but not in class B, where the overriding member is a val instead of a def.

[odersky]

Fixed in latest #339. I have added your example as a test.