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We disallow overriding of val parameters, which fixes the soundness problem discovered in scala#16092. There is one exception: If a val parameter is overridden by another val parameter that can be shown to always have the same value (in the sense established by Paramforwarding.inheritedAccessor). This exception is needed to make a not-so-uncommon pattern of case class inheritance go through. Example: abstract class A(val x: Int) case class B(override val x: Int) extends A(x) case class C(override val x: Int) extends A(x) case object D extends A(0) Here, the `override val`s are necessary since case class parameters are always vals, so they do override the val in class A. It should be noted that the override val generates a second field, so this not a very efficient representation. A better design would be to use an abstract field in `A`: abstract class A { val x: Int } case class B(val x: Int) extends A case class C(val x: Int) extends A case object D extends A { val a = 0 } But that causes slightly more work for cases as in D. Which seems to be why the first pattern is sometimes used. It might be desirable to disallow the second pattern, but that would cause quite a bit of migration hassle since it requires synchronized changes at several places of a class hierarchy.
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trait Pet(val name: String, rest: Int): | ||
def f(suffix: String) = s"$name$suffix$rest" | ||
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class Birdie(override val name: String) extends Pet("huh", 1) // error | ||
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trait X: | ||
type T | ||
def process(t: T): Unit | ||
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abstract class Z: | ||
def x1: X | ||
val x: X = x1 | ||
def t: x.T | ||
def process(): Unit = x.process(t) | ||
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class Evil extends Z: | ||
def x2: X | ||
override val x: X = x2 | ||
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// alarm bells should be ringing by now | ||
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// taking it to its conclusion... | ||
object X1 extends X: | ||
override type T = Int | ||
override def process(t: T): Unit = println("Int: " + t) | ||
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object X2 extends X: | ||
override type T = String | ||
override def process(t: T): Unit = println("String: " + t) | ||
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@main def Test = | ||
new Evil{ | ||
val x1 = X1 | ||
val x2 = X2 | ||
val t = 42 // error | ||
}.process() // BOOM: basically did x2.process(42) |
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trait X { | ||
type T | ||
def process(t: T): Unit | ||
} | ||
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class Z(val x: X, val t: x.T) { | ||
def process(): Unit = x.process(t) | ||
} | ||
class Evil(x1: X, x2: X, t: x1.T) extends Z(x1, t) { | ||
val x: X = x2 // error breaks connection between x and t | ||
} | ||
// alarm bells should be ringing by now | ||
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// taking it to its conclusion... | ||
object x1 extends X { | ||
override type T = Int | ||
override def process(t: T): Unit = println("Int: " + t) | ||
} | ||
object x2 extends X { | ||
override type T = String | ||
override def process(t: T): Unit = println("String: " + t) | ||
} | ||
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@main def Test = new Evil(x1, x2, 42).process() // BOOM: basically did x2.process(42) |
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@@ -1,4 +1,4 @@ | ||
trait A(val s: String) { println(s) } | ||
trait B extends A { override val s = "B" } // requires override val s | ||
trait A(s: String) { println(s) } | ||
trait B extends A { val s = "B" } | ||
class C extends B // error | ||
@main def Test = C() |
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Original file line number | Diff line number | Diff line change |
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class A(val x: Int) | ||
class B(override val x: Int) extends A(x) | ||
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class C(x: Int) extends A(x) | ||
case class D(override val x: Int) extends C(x) |
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class A(a: Int) | ||
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class B extends A(1): | ||
val a = 2 // ok | ||
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@main def Test = | ||
assert(B().a == 2) |
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