Fix #2971: Soudness issue with variance and higher kinded types

We shouldn't use `HKApply#superType` in `isRef`, because this means
taking the upper bound of abstract type constructors.

Author: smarter

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -132,7 +132,7 @@ object Types {
           case _ =>  this1.symbol eq sym
         }
       case this1: RefinedOrRecType => this1.parent.isRef(sym)
-      case this1: HKApply => this1.superType.isRef(sym)
+      case this1: HKApply => this1.underlying.isRef(sym)
       case _ => false
     }
 

tests/pos/i2971.scala

@@ -0,0 +1,20 @@
+case class Foo[+X[_]](will: X[Int]) {
+  def foo[Y[_]](right: Foo[Y]) = Foo.doFoo(this, right)
+}
+
+class A[X] { def crash = true }
+class B[X]
+
+object Foo {
+  def doFoo[X[_]](left: Foo[X], right: Foo[X]): Foo[X] = right
+
+  def main(args: Array[String]): Unit = {
+    val fooA = Foo(new A[Int])
+    val fooB = Foo(new B[Int])
+    // The type for this is inferred correctly to Foo[A|B]
+    val fine = doFoo(fooA, fooB)
+    // This throws a ClassCastException because fooB isn't a Foo[A]
+    val oops: Foo[A] = fooA.foo(fooB) // error: found: Foo[B], required: Foo[A]
+    println(oops.will.crash)
+  }
+}