type A[X] = X match case Int => Int case _ => B[X] def a[X](x: X): A[X] = x match case v: Int => v case _ => b(x) type B[X] = X match case String => String def b[X](x: X): B[X] = x match case v: String => v

-- [E007] Type Mismatch Error: foo.scala:6:17 ---------------------------------- 6 | case v: Int => v | ^ | Found: (v : X & Int) | Required: A[X] | | Note: a match type could not be fully reduced: | | trying to reduce A[X] | failed since selector X | does not match case Int => Int | and cannot be shown to be disjoint from it either. | Therefore, reduction cannot advance to the remaining case | | case _ => B[X] longer explanation available when compiling with `-explain` -- [E007] Type Mismatch Error: foo.scala:7:18 ---------------------------------- 7 | case _ => b(x) | ^^^^ | Found: B[X] | Required: A[X] | | Note: a match type could not be fully reduced: | | trying to reduce B[X] | failed since selector X | does not match case String => String | and cannot be shown to be disjoint from it either. longer explanation available when compiling with `-explain` 2 errors found

type A[X] = X match case Int => Int case _ => B[X] def a[X](x: X): A[X] = (x match case v: Int => v case _ => b(x) ).asInstanceOf[A[X]] type B[X] = X match case String => String def b[X](x: X): B[X] = x match case v: String => v