Merge pull request #9565 from dotty-staging/fix-9473

Fix #9473: Add test

Author: liufengyun

docs/docs/reference/contextual/derivation.md

@@ -262,15 +262,15 @@ object Eq {
 
   def iterator[T](p: T) = p.asInstanceOf[Product].productIterator
 
-  def eqSum[T](s: Mirror.SumOf[T], elems: List[Eq[_]]): Eq[T] =
+  def eqSum[T](s: Mirror.SumOf[T], elems: => List[Eq[_]]): Eq[T] =
     new Eq[T] {
       def eqv(x: T, y: T): Boolean = {
         val ordx = s.ordinal(x)
         (s.ordinal(y) == ordx) && check(elems(ordx))(x, y)
       }
     }
 
-  def eqProduct[T](p: Mirror.ProductOf[T], elems: List[Eq[_]]): Eq[T] =
+  def eqProduct[T](p: Mirror.ProductOf[T], elems: => List[Eq[_]]): Eq[T] =
     new Eq[T] {
       def eqv(x: T, y: T): Boolean =
         iterator(x).zip(iterator(y)).zip(elems.iterator).forall {
@@ -279,7 +279,7 @@ object Eq {
     }
 
   inline given derived[T](using m: Mirror.Of[T]) as Eq[T] = {
-    val elemInstances = summonAll[m.MirroredElemTypes]
+    lazy val elemInstances = summonAll[m.MirroredElemTypes]
     inline m match {
       case s: Mirror.SumOf[T]     => eqSum(s, elemInstances)
       case p: Mirror.ProductOf[T] => eqProduct(p, elemInstances)

tests/run/i9473.scala

@@ -0,0 +1,59 @@
+import scala.deriving._
+import scala.compiletime.{erasedValue, summonInline}
+
+inline def summonAll[T <: Tuple]: List[Eq[_]] = inline erasedValue[T] match {
+  case _: EmptyTuple => Nil
+  case _: (t *: ts) => summonInline[Eq[t]] :: summonAll[ts]
+}
+
+trait Eq[T] {
+  def eqv(x: T, y: T): Boolean
+}
+
+object Eq {
+  given Eq[Int] {
+    def eqv(x: Int, y: Int) = x == y
+  }
+
+  def check(elem: Eq[_])(x: Any, y: Any): Boolean =
+    elem.asInstanceOf[Eq[Any]].eqv(x, y)
+
+  def iterator[T](p: T) = p.asInstanceOf[Product].productIterator
+
+  def eqSum[T](s: Mirror.SumOf[T], elems: => List[Eq[_]]): Eq[T] =
+    new Eq[T] {
+      def eqv(x: T, y: T): Boolean = {
+        val ordx = s.ordinal(x)
+        (s.ordinal(y) == ordx) && check(elems(ordx))(x, y)
+      }
+    }
+
+  def eqProduct[T](p: Mirror.ProductOf[T], elems: => List[Eq[_]]): Eq[T] =
+    new Eq[T] {
+      def eqv(x: T, y: T): Boolean =
+        iterator(x).zip(iterator(y)).zip(elems.iterator).forall {
+          case ((x, y), elem) => check(elem)(x, y)
+        }
+    }
+
+  inline given derived[T](using m: Mirror.Of[T]) as Eq[T] = {
+    lazy val elemInstances = summonAll[m.MirroredElemTypes]
+    inline m match {
+      case s: Mirror.SumOf[T]     => eqSum(s, elemInstances)
+      case p: Mirror.ProductOf[T] => eqProduct(p, elemInstances)
+    }
+  }
+}
+
+enum Tree[T] derives Eq {
+  case Branch(left: Tree[T], right: Tree[T])
+  case Leaf(elem: T)
+}
+
+@main
+def Test = {
+  import Tree._
+
+  val t1 = Branch(Leaf(1), Leaf(1))
+  assert(summon[Eq[Tree[Int]]].eqv(t1, t1))
+}