Add fields sorting (primitives first) in case class equals

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

@@ -161,7 +161,9 @@ class SyntheticMethods(thisPhase: DenotTransformer) {
       val thatAsClazz = ctx.newSymbol(ctx.owner, nme.x_0, Synthetic, clazzType, coord = ctx.owner.pos) // x$0
       def wildcardAscription(tp: Type) = Typed(Underscore(tp), TypeTree(tp))
       val pattern = Bind(thatAsClazz, wildcardAscription(AnnotatedType(clazzType, Annotation(defn.UncheckedAnnot)))) // x$0 @ (_: C @unchecked)
-      val comparisons = accessors map { accessor =>
+      // compare primitive fields first, slow equality checks of non-primitive fields can be skipped when primitives differ
+      val sortedAccessors = accessors.sortBy(accessor => if (accessor.info.typeSymbol.isPrimitiveValueClass) 0 else 1)
+      val comparisons = sortedAccessors.map { 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 _)

tests/run/case_class_equals_fields_sort.scala

@@ -0,0 +1,9 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    class X { override def equals(x: Any) = throw new Exception("shouldn't be called") }
+    case class C(x: X, i: Int)
+    val x = new X
+
+    C(x, 1) == C(x, 2)
+  }
+}