Scala 3：处理路径相关类型

Question

Consider the following snippet:

object Test extends App {
  class X {
    class Y
  }

  class Z(val x: X) {
    val y: x.Y = new x.Y
  }

  val x: X = new X
  val z: Z = new Z(x)
  val y: x.Y = z.y

  println(y)
}

This code won't compile, complaing about incompatible path-dependent types:

[error] 12 |  val y: x.Y = z.y
[error]    |               ^^^
[error]    |               Found:    (Test.z.y : Test.z.x.Y)
[error]    |               Required: Test.x².Y
[error]    |
[error]    |               where:    x  is a value in class Z
[error]    |                         x² is a value in object Test
[error]    |

Is there a way to gently remind the compiler that zx is assigned to x just one line above?

Even if (zx == x) { val y: xY = zy } does not solve the issue, even though path equivalence should be inferred from control flow.

Background: Scala 3 macro API heavily uses PDT's, and this creates huge amount of pain to deal with --- all this pain is coming from compiler's inability to infer anything, and lack of syntactic structures to explicitly control that inference.

Answer 1

This is correct behavior for path-dependent types (by the way, in Scala 2 it's the same).

Would you be satisfied with

val y: z.x.Y = z.y // compiles

?

zx equals x but this doesn't mean that the type zxY is xY .

Similarly,

class A {
  type T
}
val a = new A
val a1 = a
//implicitly[a.T =:= a1.T] // doesn't compile

a equals to a1 but this doesn't mean that the type aT is a1.T .

This is Scala 2 spec for equivalence of path-dependent types: https://scala-lang.org/files/archive/spec/2.13/03-types.html#equivalence . In our case the prefixes have different singleton types:

implicitly[z.x.type =:= x.type] // doesn't compile

• If a path p has a singleton type q.type , then p.type ≡ q.type .
• If O is defined by an object definition, and p is a path consisting only of package or object selectors and ending in O , then O.this.type ≡ p.type .

You can fix the compilation:

class A {
  type T
}
val a = new A
val a1: a.type = a
implicitly[a.T =:= a1.T] // compiles

and

class X {
  class Y
}

class Z(val x: X) {
  type V = x.Y // added
  val y: V = new x.Y
}

val x: X = new X
val z: Z = new Z(x)
val y: z.V = z.y // compiles

See also:

In the latest release of scala (2.12.x), is the implementation of path-dependent type incomplete?

Cannot prove equivalence with a path dependent type

Force dependent types resolution for implicit calls

How to help the Scala 3 compiler infer a path-dependent-type?

How to create an instances for typeclass with dependent type using shapeless

If you use REPL you'll see the types inferred by the compiler (removing explicit type annotations as @LuisMiguelMejíaSuárez advised in comments):

scala> class X {
     |   class Y
     | }
class X

scala> class Z(val x: X) {
     |   val y: x.Y = new x.Y
     | }
class Z

scala> val x = new X
val x: X = X@6f76c2cc

scala> val z = new Z(x)
val z: Z = Z@7e62cfa3

scala> val y = z.y
val y: z.x.Y = X$Y@52bd9a27  // notice z.x, not x