Return type of method not checked when overriding methods from self type in trait

When overriding a method from the self type of a trait the return type is not checked so this compiles:

trait A {
    def a: Int
}
trait B { self: A =>
    override def a: String = "Well this should not compile"
}

The compile error happens when B is used:

scala> class C extends A with B
<console>:9: error: overriding method a in trait A of type => Int;
 method a in trait B of type => String has incompatible type
       class C extends A with B
             ^

The compile error should already happen when defining B as the method overrides nothing.

scala / bug

Return type of method not checked when overriding methods from self type in trait #9736