Compare productPrefix in synthetic case class canEqual

Since 2.13, case class `hashCode` mixes in the hash code of the
`productPrefix` string. The synthetic `equals` method has fallen
out of sync in that regard, so two instances can be equal but have
different hash codes.

This commit changes the synthetic `canEqual` method to also compare
the `productPrefix` of the two instances.

compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala

@@ -419,7 +419,13 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
      *
      *  `@unchecked` is needed for parametric case classes.
      */
-    def canEqualBody(that: Tree, span: Span): Tree = that.isInstance(AnnotatedType(clazzType, Annotation(defn.UncheckedAnnot, span)))
+    def canEqualBody(that: Tree, span: Span): Tree = {
+      val clazzTypeU = AnnotatedType(clazzType, Annotation(defn.UncheckedAnnot, span))
+      that.isInstance(clazzTypeU).and(
+        This(clazz).select(defn.Product_productPrefix)
+          .select(defn.Any_==)
+          .appliedTo(that.cast(clazzTypeU).select(defn.Product_productPrefix)))
+    }
 
     symbolsToSynthesize.flatMap(syntheticDefIfMissing)
   }

tests/run/t13033.scala

@@ -0,0 +1,24 @@
+case class C(x: Int)
+class D extends C(1) { override def productPrefix = "D" }
+
+abstract case class C1(a: Int)
+class C2(a: Int) extends C1(a) { override def productPrefix = "C2" }
+class C3(a: Int) extends C1(a) { override def productPrefix = "C3" }
+
+case class VCC(x: Int) extends AnyVal
+
+object Test extends App {
+  val c = C(1)
+  assert(c != new D)
+  assert(c == C(1))
+  assert(!c.canEqual(new D))
+
+  val c2 = new C2(1)
+  val c3 = new C3(1)
+  assert(c2 != c3)
+  assert(c2.hashCode != c3.hashCode)
+  assert(!c2.canEqual(c3))
+
+  assert(VCC(1).canEqual(VCC(1)))
+  assert(!VCC(1).canEqual(1))
+}