Backport "Add missing span to synthesized product mirror" to LTS (#20612

)

Backports #18354 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -429,7 +429,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         if cls.useCompanionAsProductMirror then companionPath(mirroredType, span)
         else if defn.isTupleClass(cls) then newTupleMirror(typeElems.size) // TODO: cls == defn.PairClass when > 22
         else anonymousMirror(monoType, MirrorImpl.OfProduct(pre), span)
-      withNoErrors(mirrorRef.cast(mirrorType))
+      withNoErrors(mirrorRef.cast(mirrorType).withSpan(span))
     end makeProductMirror
 
     MirrorSource.reduce(mirroredType) match
@@ -442,12 +442,12 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
               mirrorCore(defn.Mirror_SingletonProxyClass, mirroredType, mirroredType, singleton.name)
             }
             val mirrorRef = New(defn.Mirror_SingletonProxyClass.typeRef, singletonPath :: Nil)
-            withNoErrors(mirrorRef.cast(mirrorType))
+            withNoErrors(mirrorRef.cast(mirrorType).withSpan(span))
           else
             val mirrorType = formal.constrained_& {
               mirrorCore(defn.Mirror_SingletonClass, mirroredType, mirroredType, singleton.name)
             }
-            withNoErrors(singletonPath.cast(mirrorType))
+            withNoErrors(singletonPath.cast(mirrorType).withSpan(span))
         case MirrorSource.GenericTuple(tps) =>
           val maxArity = Definitions.MaxTupleArity
           val arity = tps.size

tests/pos-macros/i18353/Macro_1.scala

@@ -0,0 +1,64 @@
+import scala.compiletime.*
+import scala.deriving.*
+import scala.quoted.*
+
+trait Getter[S, A]:
+  def view: S => A
+
+trait Lens[S, A] extends Getter[S, A]:
+  def set: S => A => S
+
+object Lens {
+  inline def apply[S, A](_view: S => A)(_set: S => A => S): Lens[S, A] =
+    new Lens[S, A]:
+      def view: S => A = _view
+      def set: S => A => S = _set
+
+  inline given derived[T <: Product, A]: Lens[T, A] = ${
+    ProductMacros.genLens[T, A]
+  }
+}
+
+object ProductMacros {
+  private def indexOf[T: Type, A: Type](using Quotes): Int =
+    indexOf0[T, A](0)
+
+  private def indexOf0[T: Type, A: Type](acc: Int)(using Quotes): Int =
+    Type.of[T] match
+      case '[EmptyTuple]  => -1
+      case '[A *: tpes]   => acc
+      case '[tpe *: tpes] => indexOf0[tpes, A](acc + 1)
+
+  def genLens[T <: Product: Type, A: Type](using
+      q: Quotes
+  ): Expr[Lens[T, A]] = {
+    import quotes.reflect.*
+
+    Expr
+      .summon[Mirror.ProductOf[T]]
+      .map {
+        case '{
+              $m: Mirror.ProductOf[T] { type MirroredElemTypes = elementTypes }
+            } =>
+          val i = indexOf[elementTypes, A]
+          if i < 0 then
+            report.errorAndAbort(s"has no the field of ${Type.show[A]}")
+          else
+            val ii: Expr[Int] = Expr(i)
+            val view: Expr[T => A] = '{ t =>
+              t.productElement($ii).asInstanceOf[A]
+            }
+            val set: Expr[T => A => T] = '{ t => a =>
+              val arr = Tuple.fromProduct(t).toArray
+              arr($ii) = a.asInstanceOf[Object]
+              // Check-macros fails here probably
+              $m.fromTuple(Tuple.fromArray(arr).asInstanceOf[elementTypes])
+            }
+            '{ Lens[T, A]($view)($set) }
+      }
+      .getOrElse(
+        report.errorAndAbort(s"${Type.show[T]} is not a product type")
+      )
+
+  }
+}

tests/pos-macros/i18353/Test_2.scala

@@ -0,0 +1,7 @@
+def Test = {
+
+  type TupleConfig = (Int, String)
+
+  val tConfig = (1, "string")
+  val fails = summon[Lens[TupleConfig, Int]].view(tConfig)
+}