Unsoundness in overriding methods with higher-order type parameters

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Try to compile the following code with Scala 2.7.5 final:

trait A {
 def f[T[_]](x : T[Int]) : T[Any]
}

class B extends A {
 def f[T[+_]](x : T[Int]) : T[Any] = x
}

class P[Y](var y : Y)

It is compiled without any errors. But then you can write:

val p = new P(1)
val palias = (new B():A).f[P](p)
palias.y = "hello"

Result:

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer

So, this way of overriding is unsound.

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Imported From: https://issues.scala-lang.org/browse/SI-2066?orig=1
Reporter: Vladimir Reshetnikov (nikov)
See #5517

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Vladimir Reshetnikov (nikov) said:
Similar issue:

class C[T](var y : T)

object A {
val a : AnyRef { def f[T[_]](x : T[int]) : T[Any] } = A
def f[T[+_]](x : T[Int]) : T[Any] = x
}

Scala 2.8.0.r18050-b20090618020144 compiler compiles this without
errors. But it seems unsound, as we can write:

scala> val c = new C(1)
c: C[Int] = C@9e7d46

scala> val c1 = A.a.fC
c1: C[Any] = C@9e7d46

scala> c1.y = ""

scala> c.y
java.lang.ClassCastException: java.lang.String cannot be cast to
java.lang.Integer

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@soc said:
Behavior has changed, both examples compile in 2.9.0.r24324-b20110222020042 and don't throw at runtime.

Should the compiler reject those examples right at the beginning or can this considered to be fixed?

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@harrah said:
To the original example, add:

val z: Int = p.y

This will throw a !ClassCastException.

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@heathermiller said:
B.f should not be able to implement A.f because the type parameter of T is covariant in B but not in A. This seems like it shouldn't compile.

Added a test case for this one (pull request)

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@adriaanm said:
fixing this will break existing code -- the rule is only to do that in major versions,
so this will have to wait for 2.11

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@gkossakowski said:
Unassigning and rescheduling to M7 as previous deadline was missed.

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@retronym said:
Here's an patch that I'm cobbling together:

https://github.com/retronym/scala/compare/ticket;2066?expand=1

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@retronym said:
scala/scala#3184