Unused type parameter of kind * -> * doesn't allow proving subtyping of other type parameters #15677
Description
Compiler version
Compiles correctly for 3.0.0, 3.0.1
Gives an error for 3.0.2
Compiles correctly for 3.1.0, 3.1.1
Gives an error for 3.1.2, 3.1.3, 3.2.0-RC1, 3.2.1-RC1-bin-20220712-7e20b81-NIGHTLY
Minimized code
class Interval[E, D[_]]
class BoundedAbove[E, D[_]] extends Interval[E, D] {
def upper: Upper[E] = ???
}
class Upper[E]
def reference: Interval[Int, Option] = ???
def bound: Upper[Int] =
reference match {
case int: BoundedAbove[_, _] => int.upper
case _ => null
}Output
[error] ./main.scala:13:37: Found: Upper[_]
[error] Required: Upper[Int]
[error] case int: BoundedAbove[_, _] => int.upper
[error] ^^^^^^^^^Expectation
Compile correctly.
It should be possible to prove that int.upper.type <:< Upper[Int]
More information
Interestingly, the following piece of code compiles correctly. (If we change the kind of D to * or remove it altogether)
class Interval[E, D]
class BoundedAbove[E, D] extends Interval[E, D] {
def upper: Upper[E] = ???
}
class Upper[E]
def reference: Interval[Int, String] = ???
def bound: Upper[Int] =
reference match {
case int: BoundedAbove[_, _] => int.upper
case _ => null
}