Fix #3140: Generate better equals for case/value classes

- Do overloading resolution on `==` to avoid boxing primitives
- Use `case _` instead of `case _: Any` for the default case, the latter
  produces suboptimal code.

Author: smarter

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

@@ -164,11 +164,11 @@ class SyntheticMethods(thisPhase: DenotTransformer) {
       def wildcardAscription(tp: Type) = Typed(Underscore(tp), TypeTree(tp))
       val pattern = Bind(thatAsClazz, wildcardAscription(clazzType)) // x$0 @ (_: C)
       val comparisons = accessors map { accessor =>
-        This(clazz).select(accessor).select(defn.Any_==).appliedTo(ref(thatAsClazz).select(accessor)) }
+        This(clazz).select(accessor).equal(ref(thatAsClazz).select(accessor)) }
       val rhs = // this.x == this$0.x && this.y == x$0.y
         if (comparisons.isEmpty) Literal(Constant(true)) else comparisons.reduceLeft(_ and _)
       val matchingCase = CaseDef(pattern, EmptyTree, rhs) // case x$0 @ (_: C) => this.x == this$0.x && this.y == x$0.y
-      val defaultCase = CaseDef(wildcardAscription(defn.AnyType), EmptyTree, Literal(Constant(false))) // case _ => false
+      val defaultCase = CaseDef(Underscore(defn.AnyType), EmptyTree, Literal(Constant(false))) // case _ => false
       val matchExpr = Match(that, List(matchingCase, defaultCase))
       if (isDerivedValueClass(clazz)) matchExpr
       else {

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

@@ -241,4 +241,27 @@ class TestBCode extends DottyBytecodeTest {
       assert(!hasInstanceof, "Try case should not issue INSTANCEOF opcode\n")
     }
   }
+
+  @Test def noBoxingInSyntheticEquals = {
+    val source =
+      """
+        |case class Case(x: Long)
+        |class Value(val x: Long) extends AnyVal
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      for ((clsName, methodName) <- List(("Case", "equals"), ("Value$", "equals$extension"))) {
+        val moduleIn = dir.lookupName(s"$clsName.class", directory = false)
+        val moduleNode = loadClassNode(moduleIn.input)
+        val equalsMethod = getMethod(moduleNode, methodName)
+
+        val callsEquals = instructionsFromMethod(equalsMethod).exists {
+            case i @ Invoke(_, _, "equals", _, _) => true
+            case i => false
+        }
+
+        assert(!callsEquals, s"equals method should not be called in the definition of $clsName#$methodName\n")
+      }
+    }
+  }
 }