this qualified field masked by argument

[scabug]

I can't say definitively this is a bug. But I think it should be. There is only one thing which "this.x" can mean. It looks to me like this could be implemented with some extra logic in typedAssign.

class A(var x: Int)
class B(x: Int) extends A(x) { def update(z: Int) = this.x = z }

a.scala:2: error: reassignment to val
class B(x: Int) extends A(x) { def update(z: Int) = this.x = z }
                                                           ^
one error found

I would expect it to work like the common situation (common in java anyway) where one does:

def f(x: Int) = this.x = x

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3194?orig=1
Reporter: @paulp

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@odersky said:
I think this is fundamentally a duplicate of another ticket, but I can't find it right now. The problem is that the constructor parameter does get modelled as a private[this] field. For the duplicate I decided that this should wait until after 2.8 since changing it now would be too risky.

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@paulp said:
Replying to [comment:3 odersky]:

I think this is fundamentally a duplicate of another ticket, but I can't find it right now. The problem is that the constructor parameter does get modelled as a private[this] field. For the duplicate I decided that this should wait until after 2.8 since changing it now would be too risky.

That's fine - there's no ticket I open these days intending it to be addressed before 2.8 unless it involves planes falling out of the sky. I only open them so I don't forget.

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@paulp said:
See #3861 for a related issue and a workaround to this one. (TL/DR: give B a self-type of A.)

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@paulp said:
Here is another related issue. There does not seem to be any way to get this to compile without changing the name of the parameter in Foo.

trait Settings {
  type T = Int
  def f: T = 5
}

class Bar(val settings: Settings) { }

class Foo(settings: Settings) extends Bar(settings) {
  def g = settings.f
}

a.scala:9: error: private value settings escapes its defining scope as part of type Foo.this.settings.T
  def g = settings.f
      ^
one error found

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@paulp said:
Replying to [comment:5 extempore]:

See #3861 for a related issue and a workaround to this one. (TL/DR: give B a self-type of A.)

Now that #3861 is fixed, as one might suppose, the workaround no longer works around.

// This code compiles in 2.8.1 but not since whenever SI-3861 was fixed
// (tested at r24303.)
class A(var x: Int)
class B(x: Int) extends A(x) {
  self: A =>

  def update(z: Int) = this.x = z
}

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@paulp said:
See also #4606.

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@paulp said:
See also #4762.

[som-snytt]

The limited fix is in the linked PR. We are safely past 2.8 final, so the risk of the change is deemed acceptable. There are variants mentioned here that are worth revisiting.