Spurious "illegal trait inheritance"

[oyvindberg]

Compiler version

3.0.0

Minimized code

The two first types are a minimization of scalajs-library, making this code independent of scalajs.
The last five form a somewhat interesting inheritance chain.

trait JsAny extends AnyRef
class JsObject extends JsAny

trait HTMLAttributes[T] extends JsObject
trait Component[P] extends JsObject
trait IPersonaSharedProps extends HTMLAttributes[PersonaCoinBase]
trait PersonaCoinBase extends Component[IPersonaCoinProps]
trait IPersonaCoinProps extends IPersonaSharedProps

Output

-- Error: tester.scala:8:32 
8 |trait IPersonaCoinProps extends IPersonaSharedProps
  |                                ^^^^^^^^^^^^^^^^^^^
  |illegal trait inheritance: superclass Object does not derive from trait IPersonaSharedProps's superclass JsObject
1 error found

Expectation

Compiles, it did in Scala 2.

Most changes to the code above removes the compile error.

[oyvindberg]

Most changes to the code above removes the compile error.

I should add that I added this phrase to highlight that it takes a fairly specific circumstance to trigger this, not that it's insignificant. It was a huge pain to discover, try to understand and minimize. I've also yet to discover a workaround for it.

With this bug showing up for this concrete case, I suppose it's likely that other trait composition constellations are affected as well

[som-snytt]

With shorter names for visual clarity

//trait JsAny extends AnyRef
class JsObject //extends JsAny

trait HTMLAttributes[A] extends JsObject
trait Component[B] extends JsObject

trait T extends HTMLAttributes[U]
trait U extends Component[V]
trait V extends T

[smarter]

As a workaround you can make the class parent JsObject of the trait explicit

trait IPersonaCoinProps extends JsObject with IPersonaSharedProps

[oyvindberg]

thanks a lot @smarter I'll try it

[odersky]

The method to investigate further here is typedParentTypes and in particular normalizeFirstParent.

[odersky]

It's a compilation order problem.The parents of a class were not evaluated yet at the point where we need the info. This will be quite hard to fix without some additional lazy datastructures