Let traits with given clauses be implemented indirectly

[travisbrown]

Suppose you have a hierarchy like this:

trait Foo[A]
trait Bar[A] extends Foo[A]
trait Baz[A] extends Bar[A]

trait FooLaws[A] given Foo[A]
trait BarLaws[A] given Bar[A] extends FooLaws[A]
trait BazLaws[A] given Baz[A] extends BarLaws[A]

It'd be nice to be able to write something like this:

def instance[A] given Baz[A]: BazLaws[A] = new BazLaws[A] {}

Instead of:

def instance[A] given Baz[A]: BazLaws[A] =
  new BazLaws[A] with BarLaws[A] with FooLaws[A] {}

The former fails on the current nightly with:

1 |def instance[A] given Baz[A]: BazLaws[A] = new BazLaws[A] {}
  |                                           ^
  |          parameterized trait FooLaws is indirectly implemented,
  |          needs to be implemented directly so that arguments can be passed

[smarter]

SIP-25 says "Only classes can pass arguments to parent traits. Traits themselves can pass arguments to neither classes nor traits." and this is pretty central to making trait constructors work I think, so BarLaws can't pass its Bar[A] to FooLaws. However what we could maybe do is: when we see that there's an indirectly implemented trait whose constructor parameters haven't been filled and these parameters are all implicit, perform an implicit search before failing with the error you're seeing.

[soronpo]

I also proposed something similar on the SIP's contributor's thread https://contributors.scala-lang.org/t/proposal-to-add-trait-parameters-to-the-language/2356