Expansion of wildcard to existential type ignores bound

[scabug]

Given the following:

abstract class A[T <: A[T]] {
  def foo: X[T]
}

class B extends A[B] {
  def foo = new X[B]
} 

class X[T <: A[T]]

object Test {
  def test: X[_] = {
    val a: A[_] = new B

    def f[T <: A[T]](x: X[T]) = x

    f(a.foo)
  }
}

compilation fails as:

Test.scala:18: error: inferred type arguments [_$$2] do not conform to method f's type parameter bounds [T <: A[T]]
    f(a.foo)
    ^

even though it is not possible for any instance of A to fail to satisfy this bound. Explicitly specifying the existential type allows correct compilation:

object Test {
  def test: X[_] = {
    val a: A[T] forSome {type T <: A[T]} = new B

    def f[T <: A[T]](x: X[T]) = x

    f(a.foo)
  }
}

It seems like A[_] is naively being expanded to A[T] forSome {type T} instead of respecting the bound declared in A. Minor, but unexpected.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2385?orig=1
Reporter: Kris Nuttycombe (nuttycom)

[scabug]

@cunei said:
What you write corresponds to what is stated in the Scala specification (section "Placeholder Syntax for Existential Types").

The syntax A[_] is equivalent to A[T] forSome { type T >: scala.Nothing <: scala.Any }. When you explicitly specify val a:A[_] you are telling the compiler to assume that that one is the type of a.

You can just write val a = new B instead, and type inference will give you the specific type you are looking for.

[scabug]

Kris Nuttycombe (nuttycom) said:

The syntax A[_] is equivalent to A[T] forsome { type T >: scala.Nothing <: scala.Any }.

In this case, shouldn't the declaration

val a: A[_]

be a type error?

[scabug]

@cunei said:
No, in this case A[T] forsome { type T >: scala.Nothing <: scala.Any } only means that there is one such T, you don't know what it is. The fact that in reality it can only be within a subset of the whole set of types doesn't make the declaration any less valid.

To make an analogy, if I tell you "I will arrive on Monday, because I can only arrive on Mondays", and in turn you tell to someone else "He will arrive on some weekday", your assertion is still valid.