Fix propagating parent diff modificatino to its children

This closes #418

Author: ghostbuster91

core/src/main/scala-2/com/softwaremill/diffx/generic/DiffMagnoliaDerivation.scala

@@ -30,10 +30,16 @@ trait DiffMagnoliaDerivation {
       val lType = ctx.split(left)(a => a)
       val rType = ctx.split(right)(a => a)
       if (lType == rType) {
-        lType.typeclass(
+        val leftTypeClass = lType.typeclass
+        val contextPath = ModifyPath.Subtype(lType.typeName.owner, lType.typeName.short)
+        val modifyFromOverride = context
+          .getOverride(contextPath)
+          .map(_.asInstanceOf[leftTypeClass.type => leftTypeClass.type])
+          .getOrElse(identity[leftTypeClass.type] _)
+        modifyFromOverride(leftTypeClass)(
           lType.cast(left),
           lType.cast(right),
-          context.getNextStep(ModifyPath.Subtype(lType.typeName.owner, lType.typeName.short)).merge(context)
+          context.getNextStep(contextPath).merge(context)
         )
       } else {
         DiffResultValue(lType.typeName.full, rType.typeName.full)

core/src/main/scala-3/com/softwaremill/diffx/generic/DiffMagnoliaDerivation.scala

@@ -50,10 +50,16 @@ trait DiffMagnoliaDerivation extends Derivation[Diff] {
         val lType = ctx.choose(left)(a => a)
         val rType = ctx.choose(right)(a => a)
         if (lType.typeInfo == rType.typeInfo) {
-          lType.typeclass(
+          val leftTypeClass = lType.typeclass
+          val contextPath = ModifyPath.Subtype(lType.typeInfo.owner, lType.typeInfo.short)
+          val modifyFromOverride = context
+            .getOverride(contextPath)
+            .map(_.asInstanceOf[leftTypeClass.type => leftTypeClass.type])
+            .getOrElse(identity[leftTypeClass.type] _)
+          modifyFromOverride(leftTypeClass)(
             lType.cast(left),
             lType.cast(right),
-            context.getNextStep(ModifyPath.Subtype(lType.typeInfo.owner, lType.typeInfo.short)).merge(context)
+            context.getNextStep(contextPath).merge(context)
           )
         } else {
           DiffResultValue(lType.typeInfo.full, rType.typeInfo.full)

core/src/main/scala/com/softwaremill/diffx/DiffContext.scala

@@ -22,7 +22,7 @@ case class DiffContext(
 
   private def treeOverride[T](nextPath: ModifyPath, tree: Tree[T]) = {
     tree match {
-      case Tree.Leaf(v)     => Some(v)
+      case Tree.Leaf(v)     => None
       case Tree.Node(tries) => getOverrideFromNode(nextPath, tries)
     }
   }

core/src/test/scala/com/softwaremill/diffx/test/DiffModifyIntegrationTest.scala

@@ -7,6 +7,7 @@ import org.scalatest.matchers.should.Matchers
 
 import java.time.Instant
 import java.util.UUID
+import scala.collection.immutable.ListMap
 
 class DiffModifyIntegrationTest extends AnyFlatSpec with Matchers with AutoDerivation {
   val instant: Instant = Instant.now()
@@ -293,4 +294,30 @@ class DiffModifyIntegrationTest extends AnyFlatSpec with Matchers with AutoDeriv
     val d2 = Diff[Family].modify(_.second.name).ignore.modify(_.first).ignore
     compare(f1, f2)(d2).isIdentical shouldBe true
   }
+
+  it should "allow to set custom diff to a nested case class field" in {
+    case class Address(house: Int, street: String)
+    case class Person(name: String, address: Address)
+
+    val add = Diff.summon[Address]
+    val d = Diff
+      .summon[Person]
+      .modify(_.address)
+      .setTo(add)
+
+    val a1 = Address(123, "Robin St.")
+    val a2 = Address(456, "Robin St.")
+    val p1 = Person("Mason", a1)
+    val p2 = Person("Mason", a2)
+    d(p1, p2) shouldBe DiffResultObject(
+      "Person",
+      ListMap(
+        "name" -> IdenticalValue("Mason"),
+        "address" -> DiffResultObject(
+          "Address",
+          ListMap("house" -> DiffResultValue(123, 456), "street" -> IdenticalValue("Robin St."))
+        )
+      )
+    )
+  }
 }