Dependent function types do not play well with type inference

[smarter]

object Test {
  val f: (x: Int) => Int = x => x

  def id[A, B](x: (y: A) => B) = x

  id(f)
    // -- [E007] Type Mismatch Error: try/dep.scala:6:5 -------------------------------
    // 6 |  id(f)
    //   |     ^
    //   |     found:    (x: Int) => Int(Test.f)
    //   |     required: (y: Nothing) => Any
    //   |     

  id[Int, Int](f) // OK
}

[b-studios]

@smarter Here is another example that gives "unspecified error" in the latest nightly (0.11.0-bin-20181119-0a1aed2-NIGHTLY)

trait A
  
def foo[E](f: (a: A) => (a.type, E)): E = {
  val a = new A {}
  f(a)._2
}

foo { a => (a, ()) }

it does type check when annotating the call to foo:

foo[Unit] { a => (a, ()) }

I guess this is related to the type inference example, above.

[b-studios]

@smarter I tried to understand the problem a bit better and learnt the following things:

• the unspecified error is raised here in Typer
• The check has been introduced long before path-dependent function types existed
• checking whether the following tree is fully defined fails:

  AppliedType(TypeRef(...,class Tuple2),List(TermRef(NoPrefix,val a), TypeVar(TypeParamRef(E))))

  However, this is not a problem when replacing a.type in (a: A) => (a.type, E) by A to yield (a: A) => (A, E).

That said, I guess the check might need to be improved, but I am not in the position to do that right now. :(