Specificity in contravariant implicit resolution #2974
Description
The Dotty resolution algorithm was recently ported to scalac by @milessabin in scala/scala#6037 (comment) where this came up:
trait Foo[-T]
trait Bar[-T] extends Foo[T]
object Test {
implicit val fa: Foo[Any] = ???
implicit val ba: Bar[Int] = ???
def test: Unit = {
implicitly[Foo[Int]]
}
}This fails because Bar[Int] is not considered more specific than Foo[Any], because the variance-flipping we do in isAsSpecificValueType is not thorough enough:
[log frontend] ==> isSubType Bar[Int] <:< Foo[Any]?
[log frontend] ==> isSubType Bar[Int] <:< Foo?
[log frontend] ==> isSubType Foo <:< Foo?
[log frontend] <== isSubType Foo <:< Foo = true
[log frontend] <== isSubType Bar[Int] <:< Foo = true
[log frontend] ==> hasMatchingMember(Bar[Int] . Foo$$T :? =+ Any), mbr: =- Int?
[log frontend] ==> isSubType =- Int <:< =+ Any?
[log frontend] ==> isSubType Any <:< Int?
[log frontend] <== isSubType Any <:< Int = false
[log frontend] <== isSubType =- Int <:< =+ Any = false
[log frontend] <== hasMatchingMember(Bar[Int] . Foo$$T :? =+ Any), mbr: =- Int = false
[log frontend] <== isSubType Bar[Int] <:< Foo[Any] = falsehasMatchingMember returns the original member T in Bar where the variance is not flipped (=- Int). We could add a special case in hasMatchingMember but then we might miss some other cases.
Personally, I've never really been convinced by the rationale for flipping only at the top-level, so I think the simplest solution would be to flip at all levels, then we could use a global mode.