Implicits classes not working for type aliases of types with invariance/covariance (unhelpful compiler error)

[jbwheatley]

reproduction steps

using Scala 2.13.x,

type Foo[F[_], A, B] = Kleisli[F, A, B]

val foo: Foo[Option, Int, Int] = Kleisli { i => Option(i) }

implicit class FooOps[F[_], A, B](val foo: Foo[F, A, B]) {
  def bar(a: A): F[B] = foo.run(a)
}

foo.bar(1) //does not compile

or see scastie snippet here https://scastie.scala-lang.org/uIri16dwQWWatxSnNyTBVg

problem

The compiler fails with value bar is not a member of...:

I found 2 ways of removing this error, either defining the implicit class using the the type being aliased:

implicit class FooOps2[F[_], A, B](val foo: Kleisli[F, A, B]) extends AnyVal {
  def bar2(a: A): F[B] = foo.run(a)
}

foo.bar2(1) //compiles

or by including the contravariance of Kleisli's second type param in the type alias too:

type Foo[F[_], -A, B] = Kleisli[F, A, B] // `-` included on the `A`!

val foo: Foo[Option, Int, Int] = Kleisli { i => Option(i) }

implicit class FooOps[F[_], A, B](val foo: Foo[F, A, B]) {
  def bar(a: A): F[B] = foo.run(a)
}

foo.bar(1) //compiles

The compiler error was quite unhelpful in debugging this issue.

[joroKr21]

Playground.FooOps is not a valid implicit value for Playground.foo.type => ?{def bar: ?} because:
type mismatch;
 found   : Playground.foo.type (with underlying type Playground.Foo[Option,Int,Int])
 required: Playground.Foo[[+A(in value <local Option>)]Option[A(in value <local Option>)],A(in method FooOps),Int]
    (which expands to)  Playground.Kleisli[[+A(in value <local Option>)]Option[A(in value <local Option>)],A(in method FooOps),Either[Throwable,Int]]

[SethTisue]

fixed in Scala 3, in the sense that the original code compiles, as it should, so no issue of an unhelpful error message even arises :-)