Avoid unchecked Expr casts

Author: nicolasstucki

docs/_docs/reference/contextual/derivation-macro.md

@@ -49,15 +49,16 @@ given derived[T: Type](using Quotes): Expr[Eq[T]] =
   ev match
     case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = elementTypes }} =>
       val elemInstances = summonAll[elementTypes]
-      def eqProductBody(Expr[T], Expr[T])(using Quotes): Expr[Boolean] = (x, y) =>
-        elemInstances.zipWithIndex.foldLeft(Expr(true: Boolean)) {
-          case (acc, (elem, index)) =>
-            val e1 = '{$x.asInstanceOf[Product].productElement(${Expr(index)})}
-            val e2 = '{$y.asInstanceOf[Product].productElement(${Expr(index)})}
-            '{ $acc && $elem.asInstanceOf[Eq[Any]].eqv($e1, $e2) }
-        }
-
-      '{ eqProduct((x: T, y: T) => ${eqProductBody('x, 'y)}) }
+      def eqProductBody(x: Expr[Product], y: Expr[Product])(using Quotes): Expr[Boolean] = {
+        elemInstances.zipWithIndex.foldLeft(Expr(true)) {
+          case (acc, ('{ $elem: Eq[t] }, index)) =>
+            val indexExpr = Expr(index)
+            val e1 = '{ $x.productElement($indexExpr).asInstanceOf[t] }
+            val e2 = '{ $y.productElement($indexExpr).asInstanceOf[t] }
+            '{ $acc && $elem.eqv($e1, $e2) }
+         }
+      }
+      '{ eqProduct((x: T, y: T) => ${eqProductBody('x.asExprOf[Product], 'y.asExprOf[Product])}) }
 
   // case for Mirror.ProductOf[T]
   // ...

tests/run-macros/i8007/Macro_3.scala

@@ -40,17 +40,17 @@ object Eq {
     ev match {
       case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = elementTypes }} =>
         val elemInstances = summonAll[elementTypes]
-        def eqProductBody(x: Expr[T], y: Expr[T])(using Quotes): Expr[Boolean] = {
-          elemInstances.zipWithIndex.foldLeft(Expr(true: Boolean)) {
-            case (acc, (elem, index)) =>
-              val e1 = '{$x.asInstanceOf[Product].productElement(${Expr(index)})}
-              val e2 = '{$y.asInstanceOf[Product].productElement(${Expr(index)})}
-
-              '{ $acc && $elem.asInstanceOf[Eq[Any]].eqv($e1, $e2) }
+        def eqProductBody(x: Expr[Product], y: Expr[Product])(using Quotes): Expr[Boolean] = {
+          elemInstances.zipWithIndex.foldLeft(Expr(true)) {
+            case (acc, ('{ $elem: Eq[t]}, index)) =>
+              val indexExpr = Expr(index)
+              val e1 = '{ $x.productElement($indexExpr).asInstanceOf[t] }
+              val e2 = '{ $y.productElement($indexExpr).asInstanceOf[t] }
+              '{ $acc && $elem.eqv($e1, $e2) }
           }
         }
         '{
-          eqProduct((x: T, y: T) => ${eqProductBody('x, 'y)})
+          eqProduct((x: T, y: T) => ${eqProductBody('x.asExprOf[Product], 'y.asExprOf[Product])})
         }
 
       case '{ $m: Mirror.SumOf[T] { type MirroredElemTypes = elementTypes }} =>