Closed
Description
class Foo[T](var elem: T)
object Test {
def setFirstInPair[T](pair: (Foo[T], Foo[T])) = {
pair._1.elem = pair._2.elem
}
def setFirstInList[T](list: List[Foo[T]]) = {
list(0).elem = list(1).elem
}
def test1(): Unit = {
val fooInt = new Foo(1)
val fooString = new Foo("")
val pair: (Foo[_], Foo[_]) = (fooInt, fooString)
setFirstInPair(pair) // Should be an error, like in Scala 2
println(fooInt.elem + 1) // ClassCastException in Dotty
}
def test2(): Unit = {
val fooInt = new Foo(1)
val fooString = new Foo("")
val list: List[Foo[_]] = List(fooInt, fooString)
setFirstInList(list) // Should be an error, like in Scala 2
println(fooInt.elem + 1) // ClassCastException in Dotty
}
def main(args: Array[String]): Unit = {
test1()
test2()
}
}The Scala 2 errors are:
try/sound.scala:16: error: no type parameters for method setFirstInPair: (pair: (Foo[T], Foo[T]))Unit exist so that it can be applied to arguments ((Foo[_], Foo[_]))
--- because ---
argument expression's type is not compatible with formal parameter type;
found : (Foo[_], Foo[_])
required: (Foo[?T], Foo[?T])
setFirstInPair(pair) // Should be an error, like in Scala 2
^
try/sound.scala:16: error: type mismatch;
found : (Foo[_], Foo[_])
required: (Foo[T], Foo[T])
setFirstInPair(pair) // Should be an error, like in Scala 2
^
try/sound.scala:24: error: no type parameters for method setFirstInList: (list: List[Foo[T]])Unit exist so that it can be applied to arguments (List[Foo[_]])
--- because ---
argument expression's type is not compatible with formal parameter type;
found : List[Foo[_]]
required: List[Foo[?T]]
setFirstInList(list) // Should be an error, like in Scala 2
^
try/sound.scala:24: error: type mismatch;
found : List[Foo[_]]
required: List[Foo[T]]
setFirstInList(list) // Should be an error, like in Scala 2
^Note that the following compiles fine in Scala 2, so it's not just applying a blanket ban on unifying with wildcards:
def foo[T](foo: Foo[T]) = {}
val x: Foo[_] = new Foo(1)
foo(x)I'm actually not sure how to reason about the difference in Dotty since we don't have a concept of existential types, /cc @Blaisorblade ?