Incompatible behavior between Dotty and Scala 2.12 on path-dependent types

[andres-pipicello]

The following code compiles successfully on Scala 2.12.10, but fails to do so on Dotty

object DepTest {
  trait Trait {
    case class Dependent()
  }
  object obj extends Trait
  case class Dep[T <: Trait](t: T) {
    def fun(q: t.Dependent): Unit = ???
  }
  Dep(obj).fun(obj.Dependent())
}

In order to make it compile, the type parameter of Dep must be explicitly specified, i.e. Dep[obj.type](obj)

A cross compiling working example is available at
https://github.com/andres-pipicello/dotty-issues/tree/master

These are the sbt steps to reproduce the issue, with their output

~/git/dotty-issues ⑂master* $ sbt
...
sbt:dotty-cross> ++ 2.12.10
...
sbt:dotty-cross> compile
...
[success] Total time: 3 s, completed Oct 12, 2019, 7:09:08 PM
sbt:dotty-cross> ++ 0.19.0-RC1
...
sbt:dotty-cross> compile
[info] Updating ...
[info] Done updating.
[info] Compiling 1 Scala source to /Users/apipicello/git/dotty-issues/target/scala-0.19/classes ...
[error] -- [E007] Type Mismatch Error: /Users/apipicello/git/dotty-issues/src/main/scala/DepTest.scala:9:28
[error] 9 |  Dep(obj).fun(obj.Dependent())
[error]   |               ^^^^^^^^^^^^^^^
[error]   |               Found:    DepTest.obj.Dependent
[error]   |               Required: Nothing
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 2 s, completed Oct 12, 2019, 7:09:28 PM
sbt:dotty-cross>

[odersky]

It works if we pass a singleton type for Dep:

Dep[obj.type](obj).fun(obj.Dependent())

I believe we have to require that. The reason Scala-2 works for this is that the code exploits a soundness hole in asSeenFrom.

[LPTK]

I wonder why this doesn't work in Dotty:

Dep(obj).fun(obj.Dependent())

But this works:

val d = Dep(obj)
d.fun(obj.Dependent())

This looks like a type inference bug to me.

Cc @smarter

[smarter]

We infer Dep[obj.type] in both cases, but when we don't write it explicitly and chain it with the fun call, the call to TypeOps#asSeenFrom happens before we've instantiated the type variable (we have ?T >: DepTest.obj.type <: DepTest.Trait), so we're not able to reduce t.Dependent to obj.Dependent. and instead approximate with Nothing (which is sound in contravariant position). Maybe we could do type variable instantiation inside asSeenFrom to avoid this? I don't have time right now to think this through but reopening so we can come back to it.

[ckipp01]

Testing this again:

//> using scala 3.3.1-RC1-bin-20230510-d6c643c-NIGHTLY

object DepTest {
  trait Trait {
    case class Dependent()
  }
  object obj extends Trait
  case class Dep[T <: Trait](t: T) {
    def fun(q: t.Dependent): Unit = ???
  }
  Dep(obj).fun(obj.Dependent())
}

correctly compiles. I'm to go ahead and assume that this has been fixed then and close, but feel free to report back if I'm misunderstanding something.

[ckipp01]

Actually, when trying to create a regression test, like I should have done before closing this, this does compile it seems, but fails the pickling tests. Here is the diff from the before/after test.

- .fun:((q: DepTest.Dep[DepTest.obj.type]#t.Dependent): scala.Unit)>@tests/pos/i7414.scala<223..235>
+ .fun:((q: DepTest.obj.Dependent): scala.Unit)>@tests/pos/i7414.scala<223..235>