Fix optimization in VCElideAllocations

We should not optimize

    new V(x) == new V(y)   to    x == y

if value class `V` redefines `equals`.

Author: odersky

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

@@ -12,7 +12,7 @@ import ExtensionMethods._, TreeExtractors._, ValueClasses._
  *  For a value class V defined as:
  *    class V(val underlying: U) extends AnyVal
  *  we avoid unnecessary allocations:
- *     new V(u1) == new V(u2) => u1 == u2
+ *     new V(u1) == new V(u2) => u1 == u2   provided V does not redefine `equals`
  *    (new V(u)).underlying() => u
  */
 class VCElideAllocations extends MiniPhase with IdentityDenotTransformer {
@@ -27,7 +27,9 @@ class VCElideAllocations extends MiniPhase with IdentityDenotTransformer {
       // new V(u1) == new V(u2) => u1 == u2
       // (We don't handle != because it has been eliminated by InterceptedMethods)
       case BinaryOp(NewWithArgs(tp1, List(u1)), op, NewWithArgs(tp2, List(u2)))
-      if (tp1 eq tp2) && (op eq defn.Any_==) && isDerivedValueClass(tp1.typeSymbol) =>
+      if (tp1 eq tp2) && (op eq defn.Any_==) &&
+         isDerivedValueClass(tp1.typeSymbol) &&
+         !defn.Any_equals.overridingSymbol(cls).exists =>
         // == is overloaded in primitive classes
         u1.equal(u2)
 

tests/run/vc-equals.scala

@@ -0,0 +1,16 @@
+object Test extends App {
+
+  class C(val s: Array[Int]) extends AnyVal {
+    override def equals(that: Any) = that match {
+      case that: C => s.deep == that.s.deep
+      case _ => false
+    }
+  }
+
+  val c = new C(Array(1, 2,3))
+
+  assert(c `equals` new C(Array(1, 2, 3)))
+  assert(c == (new C(Array(1, 2, 3)): Any))
+  assert(c == new C(Array(1, 2, 3)))
+
+}