Relevant prior commit: a56afcd.

Nb. I based the unit test on BytecodeTesting rather than include it in TypesTest because the contains failure doesn't show up in the latter case ... a limitation of SymbolTableForUnitTesting I don't understand. The test also exercises existentialAbstraction and isStrictlyMoreSpecific directly which are only accessible given an instance of Inferencer.

See also scala/scala-dev#207

Thanks for the pointer ... I'll run a community build when this goes green.

The community build fails with scalaz: the change produces some new implicit ambiguities ... similar situation to the SI-2712 fix.

I'm going to review this shortly by:

• sketching out a spectrum of ways we could implement Type.contains (in addition to the status quo and this patch, I could imagine a version that unpeels alias chains)
• cataloging the callers of contains
• showing how correctness of those callers would by improved or broken by the different implementations.

I've added a neg test which illustrates the sort of revealed ambiguity failures we get with this fix in place.

class Foo[F[_]]
object Foo {
  // Prior to this fix these two are ambiguous
  implicit def fooF0[F[_]]: Foo[F] = new Foo[F]
  implicit def fooF1: Foo[Option] = new Foo[Option]
}

class Bar[F[_]]
object Bar extends Bar0 {
  // Prior to this fix these two aren't selected because there is no
  // Foo[F] due to the ambiguity above
  // After this fix these two are ambiguous
  implicit def barF0[F[_]](implicit fooF: Foo[F]): Bar[F] = new Bar[F]
  implicit def barF1[F[_]](implicit fooF: Foo[F]): Bar[F] = new Bar[F]
}

trait Bar0 {
  // Prior to this fix we fall back to here
  implicit def barF2[F[_]]: Bar[F] = new Bar[F]
}

object Test {
  // Prior to this fix Bar.barF2[Option]
  // After this fix,
  // error: ambiguous implicit values:
  //   both method barF0 in object Bar of type [F[_]](implicit fooF: Foo[F])Bar[F]
  //   and method barF1 in object Bar of type [F[_]](implicit fooF: Foo[F])Bar[F]
  //   match expected type Bar[Option]
  //    implicitly[Bar[Option]]
  //              ^
  // one error found
  implicitly[Bar[Option]]
}

@adriaanm I'd be happy to target this at 2.13.x too.

Rebased to 2.13.x ...