Fix criterion when value classes have user defined equals methods

Author: odersky

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

@@ -3,7 +3,7 @@ package transform
 
 import ast.tpd
 import core._
-import Contexts._, Symbols._
+import Contexts._, Symbols._, Types._, Flags._, Phases._
 import DenotTransformers._, MegaPhase._
 import TreeExtractors._, ValueClasses._
 
@@ -23,13 +23,19 @@ class VCElideAllocations extends MiniPhase with IdentityDenotTransformer {
   override def runsAfter: Set[String] = Set(ElimErasedValueType.name)
 
   override def transformApply(tree: Apply)(using Context): Tree =
+    def hasUserDefinedEquals(tp: Type): Boolean =
+      val eql = atPhase(erasurePhase) {
+        defn.Any_equals.matchingMember(tp.typeSymbol.thisType)
+      }
+      eql.owner != defn.AnyClass && !eql.is(Synthetic)
+
     tree match {
-      // new V(u1) == new V(u2) => u1 == u2
+      // new V(u1) == new V(u2) => u1 == u2, unless V defines its own equals.
       // (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) &&
-         !defn.Any_equals.overridingSymbol(tp1.typeSymbol.asClass).exists =>
+      if (tp1 eq tp2) && (op eq defn.Any_==)
+         && isDerivedValueClass(tp1.typeSymbol)
+         && !hasUserDefinedEquals(tp1) =>
         // == is overloaded in primitive classes
         u1.equal(u2)
 

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -976,6 +976,35 @@ class TestBCode extends DottyBytecodeTest {
       assert((getMethod(c, "x_$eq").access & Opcodes.ACC_DEPRECATED) != 0)
     }
   }
+
+  @Test def vcElideAllocations = {
+    val source =
+      s"""class ApproxState(val bits: Int) extends AnyVal
+         |class Foo {
+         |  val FreshApprox: ApproxState = new ApproxState(4)
+         |  var approx: ApproxState = FreshApprox
+         |  def meth: Boolean = approx == FreshApprox
+         |}
+         """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Foo.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+      val meth       = getMethod(clsNode, "meth")
+
+      val instructions = instructionsFromMethod(meth)
+      val isFrameLine = (x: Instruction) => x.isInstanceOf[FrameEntry] || x.isInstanceOf[LineNumber]
+      // No allocations of ApproxState
+      assertSameCode(instructions.filterNot(isFrameLine), List(
+        VarOp(ALOAD, 0), Invoke(INVOKEVIRTUAL, "Foo", "approx", "()I", false),
+        VarOp(ALOAD, 0), Invoke(INVOKEVIRTUAL, "Foo", "FreshApprox", "()I", false),
+        Jump(IF_ICMPNE, Label(7)), Op(ICONST_1),
+        Jump(GOTO, Label(10)),
+        Label(7), Op(ICONST_0),
+        Label(10), Op(IRETURN)
+      ))
+    }
+  }
 }
 
 object invocationReceiversTestCode {