ValueOf interferes with implicit resolution of covariant typeclasses of singleton types

[hobwekiva]

trait Foo[+A]
implicit def foo[A <: Singleton](implicit v: ValueOf[A]): Foo[A] = new Foo[A] { }
val s = ""
implicitly[ValueOf[s.type]] // works
implicitly[Foo[s.type]] // doesn't work

trait Foo[+A]
implicit def foo[A <: Singleton]: Foo[A] = new Foo[A] { }
val s = ""
implicitly[Foo[s.type]] // works

If Foo is invariant, implicit resolution works. It is specific to ValueOf[A] as can be shown by:

trait Foo[+A]
trait Boo[A]
implicit def boo[A <: Singleton]: Boo[A] = new Boo[A] { }
implicit def foo[A <: Singleton](implicit v: Boo[A]): Foo[A] = new Foo[A] { }

val s = ""
implicitly[Foo[s.type]] // works fine

[milessabin]

This turns out not to be an issue with ValueOf per se, but rather an artefact of type inference which is quite tricky to reproduce without ValueOf or something equivalent.

[milessabin]

Fixed in https://github.com/milessabin/scala/commit/b5d676c3ec743d625d778565e6e3c5554a394b00.