Spurious error about incompatible type signatures when override Iterable.filter

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I've attached a code sample which fails to compile. In it I override filter on Iterable with an abstract definition claiming to return a subtype of Iterable. This fails to compile with the error message

error overriding method filter;
 method filter in trait Iterable of type (((T, T)) => Boolean)Iterable[(T, T)] has incompatible type ((A) => Boolean)Iterable[A]
trait Foo[T] extends Iterable[(T, T)]{

However, the second two traits defined in this file exhibit the same overriding pattern and work fine. Additionally, if I add an implementation (say = error("foo")) to the definition of filter in Foo, the type error goes away.

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Imported From: https://issues.scala-lang.org/browse/SI-1163?orig=1
Reporter: @DRMacIver
Attachments:

• filter.scala (created on Aug 2, 2008 2:37:29 PM UTC, 252 bytes)
• filter2.scala (created on Aug 2, 2008 5:55:43 PM UTC, 175 bytes)

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@DRMacIver said:
Hm. So with Aaron Harnly's help I've narrowed it down to something which makes me fear this might be a feature rather than a bug. I'm attaching a more minimal example. Specifically the problem seems to be that Scala doesn't allow overriding a method with an implementation with an abstract method.

I feel quite strongly that this should work, but if it's by design I suppose this will require a SIP to fix.

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Aaron Harnly (aaronharnly) said:
This distills to:

trait Super[A] { def foo: Super[A] = new Super[A] {} }
trait Sub[A] extends Super[A] { override def foo: Sub[A] }

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Aaron Harnly (aaronharnly) said:
Or actually, I'm just too used to having type parameters. It just distills to:

trait Super { def foo: Super = new Super {} }
trait Sub extends Super { override def foo: Sub }

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@DRMacIver said:
Martin says this is by design. Fair enough. Would it be possible to get a less confusing error message? The current one gives no indication of what's actually the problem (that an abstract member is overriding a concrete one)

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@odersky said:
fixed in r15705