Fix #5083: Fix field references in outer accessors

[odersky]

i5083 provides an example where a field reference had NoPrefix as a parameter, which made
it fail for producing an outer path. It's not quite clear where this came from. The fix
addresses it for now.

[liufengyun]

TLDR: My suggestion here is to disallow using trait parameters as parent outers, thus reject i5803.

Scalac and dotc behave differently here. The following program is rejected by Scalac, but accepted by Dotty:

class A(a: Int) {
  class Y {
    val y = a
  }

  class Z(val o: A) extends o.Y {
    val z = a
  }
}

Scalac error message:

test.scala:6: error: not found: value o
  class Z(val o: A) extends o.Y {
                            ^
one error found

The restriction in Scalac makes all outers of an object's classes known at the instantiation point. This way, calling a method during object initialization will never result in NPE related to outers, though it may still be unsafe due to possibly uninitialized fields.

Removal of the restriction in Dotty without trait parameters is still outer-safe, because primary constructor parameters of classes are assigned early before any constructor is executed.

However, with the support for trait parameters, this invariant for outer is lost, as trait primary constructor params are initialized according to linearization. We will get an NPE in the following example:

class A(a: Int) {
  abstract class X {
    def f: Int
    val x = a + f      // NPE: the outer for `Y` is not yet set
  }

  trait Y {
    val y = a
    def f: Int = A.this.a      // NPE: the outer for `Y` is not yet set
  }

  trait Z(val o: A) extends o.Y {
    val z = a
  }

  class B extends X with Z(new A(4))
}


object Test {
  def main(args: Array[String]): Unit = {
    val a = new A(3)
    new a.B
  }
}

My suggestion here is to disallow using trait parameters as parent outers, this way we maintain the nice invariant about outers, and it can avoid subtle bugs in practice. Invariants are good, though we still don't have full initialization safety.

[odersky]

@liufengyun I think your suggestion of disallowing trait outers makes sense.