Missing requirement: self type does not conform to self type A

[olafurpg]

The following code snippet compiles with scalac but not with dotc

package squants
trait Quantity[A <: Quantity[A]] { self: A => }
trait TimeDerivative[A <: Quantity[A]] { self: Quantity[_] => }

Dotty reports the error message

-- Error: tests/pos/missing-requirement/Quantity.scala -------------------------
3 |trait TimeDerivative[A <: Quantity[A]] { self: Quantity[_] => }
  |      ^
  |missing requirement: self type squants.Quantity[_] & squants.TimeDerivative[A] of trait TimeDerivative does not conform to self type A' of required trait Quantity
  |
  |where:    A  is a type in trait TimeDerivative with bounds <: squants.Quantity[LazyRef(A)]
  |          A' is a type in trait Quantity with bounds <: squants.Quantity[LazyRef(A)]

Removing the self: Quantity[_] annotation on TimeDerivative removes the error, and makes the rest of the squants project compile with Dotty 🎉

[odersky]

We now check that the self type is closed. See 1754995
in #702. We should still spec this.

[olafurpg]

What is the recommended migration path for code like TimeDerivative above? Could the error message provide more details?

[odersky]

@olafurpg Let's worry about this if we see several errors of this type. For know I am hoping this is quite a rare case.

[smarter]

@olafurpg I think you should be able to replace self: Quantity[_] by self: A

[Blaisorblade]

For what it's worth: I'm not sure I understand https://github.com/lampepfl/dotty/pull/702/files#diff-ca3386647f5386b6f70e99f1a9e188cdR3196, but certainly the error "missing requirement" is misleading.

[SethTisue]

A simpler example that Scala 2 accepts but 3 does not came up on Gitter yesterday:

trait A
trait B { self: A => }
trait C { self: B => }

[dwijnand]

That's the example in 1754995 (in reverse line order and alphabet letter order) so I take it as expected and intentional.