fix: #1913

auto_route/lib/src/route/auto_route_config.dart

@@ -351,7 +351,7 @@ class AdaptiveRoute extends AutoRoute {
 
 /// Builds an [AutoRoute] instance with [RouteType.custom] type
 @immutable
-class CustomRoute extends AutoRoute {
+class CustomRoute<T> extends AutoRoute {
   /// Default constructor
   CustomRoute({
     required PageInfo page,
@@ -368,7 +368,7 @@ class CustomRoute extends AutoRoute {
     super.initial,
     super.allowSnapshotting = true,
     RouteTransitionsBuilder? transitionsBuilder,
-    CustomRouteBuilder? customRouteBuilder,
+    CustomRouteBuilder<T>? customRouteBuilder,
     int? durationInMilliseconds,
     int? reverseDurationInMilliseconds,
     bool opaque = true,
@@ -378,7 +378,7 @@ class CustomRoute extends AutoRoute {
     Color? barrierColor,
   }) : super._(
           name: page.name,
-          type: RouteType.custom(
+          type: RouteType<T>.custom(
             transitionsBuilder: transitionsBuilder,
             customRouteBuilder: customRouteBuilder,
             durationInMilliseconds: durationInMilliseconds,

auto_route/lib/src/route/route_type.dart

@@ -11,7 +11,7 @@ typedef CustomRouteBuilder<T> = Route<T> Function(
 
 /// An abstraction of route types used by
 /// [AutoRoutePage.onCreateRoute] to decide transition animations
-abstract class RouteType {
+abstract class RouteType<T> {
   const RouteType._({this.opaque = true});
 
   /// Whether the target [Route] should be opaque
@@ -40,7 +40,7 @@ abstract class RouteType {
   /// Builds a [CustomRouteType] route type
   const factory RouteType.custom({
     RouteTransitionsBuilder? transitionsBuilder,
-    CustomRouteBuilder? customRouteBuilder,
+    CustomRouteBuilder<T>? customRouteBuilder,
     int? durationInMilliseconds,
     int? reverseDurationInMilliseconds,
     bool opaque,
@@ -51,13 +51,13 @@ abstract class RouteType {
 }
 
 /// Generates a route that uses [MaterialRouteTransitionMixin]
-class MaterialRouteType extends RouteType {
+class MaterialRouteType<T> extends RouteType<T> {
   /// Default constructor
   const MaterialRouteType() : super._(opaque: true);
 }
 
 /// Generates a route that uses [CupertinoRouteTransitionMixin]
-class CupertinoRouteType extends RouteType {
+class CupertinoRouteType<T> extends RouteType<T> {
   /// Default constructor
   const CupertinoRouteType() : super._(opaque: true);
 }
@@ -67,13 +67,13 @@ class CupertinoRouteType extends RouteType {
 /// ios,macos => [CupertinoRouteTransitionMixin]
 /// web => NoTransition
 /// any other platform => [MaterialRouteTransitionMixin]
-class AdaptiveRouteType extends RouteType {
+class AdaptiveRouteType<T> extends RouteType<T> {
   /// Default constructor
   const AdaptiveRouteType({super.opaque}) : super._();
 }
 
 /// Generates a route with user-defined transitions
-class CustomRouteType extends RouteType {
+class CustomRouteType<T> extends RouteType<T> {
   /// this builder function is passed to the transition builder
   /// function in [PageRouteBuilder]
   ///
@@ -98,7 +98,7 @@ class CustomRouteType extends RouteType {
   /// this builder function accepts a BuildContext and a CustomPage
   /// that has all the other properties assigned to it
   /// so using them then is totally up to you.
-  final CustomRouteBuilder? customRouteBuilder;
+  final CustomRouteBuilder<T>? customRouteBuilder;
 
   /// route transition duration in milliseconds
   /// is passed to [PageRouteBuilder]

auto_route/lib/src/router/auto_route_page.dart

@@ -86,8 +86,9 @@ class AutoRoutePage<T> extends Page<T> {
       return _PageBasedCupertinoPageRoute<T>(page: this, title: title);
     } else if (type is CustomRouteType) {
       final result = buildPage(context);
-      if (type.customRouteBuilder != null) {
-        return type.customRouteBuilder!(context, result, this) as Route<T>;
+      final builder = (type as CustomRouteType<T>).customRouteBuilder;
+      if (builder != null) {
+        return builder(context, result, this);
       }
       return _CustomPageBasedPageRouteBuilder<T>(page: this, routeType: type);
     } else if (type is AdaptiveRouteType) {