Find implicitNotFoundMessage on types

[odersky]

Fixes #11797

[julienrf]

Do we want to also add a test with an opaque type? Are opaque types just represented as a TypeRef like transparent types?

[odersky]

yes that's the same

[adamgfraser]

Does this work when the type is just a type alias like this?

import scala.annotation.implicitNotFound

@implicitNotFound("${A} is not a subtype of ${B}")
type IsSubtypeOf[-A, +B] = A <:< B

object Example {
  implicitly[String IsSubtypeOf Int]
}

[odersky]

Does this work when the type is just a type alias like this?

No, since the compiler is free to dealias the type before attempting an implicit search. It has to be an abstract type or opaque type.

[adamgfraser]

Would it be possible to support that? That functionality exists on Scala 2 and can be helpful for library authors to be able to alias existing types and provide more specific error messages associated with an implicit not being found rather than having to create new data types to wrap the existing ones.

[julienrf]

It has to be an abstract type or opaque type.

How is this situation handled?

trait Foo:
  @implicitNotFound("Oops")
  type Bar

trait Foo2 extends Foo:
  type Bar = Unit

object Foo3 extends Foo2:
  summon[Bar]

Although Bar is initially defined as an abstract type member in Foo, it becomes a transparent type alias in Foo2.

[odersky]

@julienrf Looks like this falls back to the generic case as well.

[odersky]

I had a look: it does not look easy to improve on this. It seems the type is dealiased way before we try to compose an error message.

[adamgfraser]

@odersky Thanks for looking!