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Unsoundness in overriding methods with higher-order type parameters #2066
Description
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.IntegerSo, this way of overriding is unsound.
Activity
scabug commentedon Jun 17, 2009
Imported From: https://issues.scala-lang.org/browse/SI-2066?orig=1
Reporter: Vladimir Reshetnikov (nikov)
See #5517
scabug commentedon Jun 19, 2009
Vladimir Reshetnikov (nikov) said:
Similar issue:
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
scabug commentedon Mar 4, 2011
@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?
scabug commentedon Apr 7, 2011
@harrah said:
To the original example, add:
val z: Int = p.yThis will throw a !ClassCastException.
scabug commentedon May 7, 2012
@heathermiller said:
B.fshould not be able to implementA.fbecause the type parameter ofTis covariant inBbut not inA. This seems like it shouldn't compile.Added a test case for this one (pull request)
scabug commentedon Feb 3, 2013
@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
scabug commentedon Oct 15, 2013
@gkossakowski said:
Unassigning and rescheduling to M7 as previous deadline was missed.
scabug commentedon Nov 22, 2013
@retronym said:
Here's an patch that I'm cobbling together:
https://github.com/retronym/scala/compare/ticket;2066?expand=1
scabug commentedon Nov 22, 2013
@retronym said:
scala/scala#3184