From dd5a1760412e0d65bd653fbc27d27d5cf41f6c43 Mon Sep 17 00:00:00 2001
From: Matt Brophy <matt@brophy.org>
Date: Wed, 6 Aug 2025 14:41:11 -0400
Subject: [PATCH 1/2] Add support for optional segmnts in nested absolute
 routes

---
 .../__tests__/path-matching-test.tsx          | 70 +++++++++++++++++++
 packages/react-router/lib/router/utils.ts     | 23 +++++-
 2 files changed, 91 insertions(+), 2 deletions(-)

diff --git a/packages/react-router/__tests__/path-matching-test.tsx b/packages/react-router/__tests__/path-matching-test.tsx
index d13c3f0bf6..337b99673d 100644
--- a/packages/react-router/__tests__/path-matching-test.tsx
+++ b/packages/react-router/__tests__/path-matching-test.tsx
@@ -487,6 +487,76 @@ describe("path matching with optional segments", () => {
       { path: "abc", params: {} },
     ]);
   });
+
+  test("optional static segments in nested absolute routes (leading)", () => {
+    let nested = [
+      {
+        path: "/en?",
+        children: [
+          {
+            path: "/en?/abc",
+            children: [
+              {
+                path: "/en?/abc/def",
+              },
+            ],
+          },
+        ],
+      },
+    ];
+
+    expect(pickPathsAndParams(nested, "/en/abc")).toEqual([
+      { path: "/en?", params: {} },
+      { path: "/en?/abc", params: {} },
+    ]);
+    expect(pickPathsAndParams(nested, "/abc")).toEqual([
+      { path: "/en?", params: {} },
+      { path: "/en?/abc", params: {} },
+    ]);
+    expect(pickPathsAndParams(nested, "/en/abc/def")).toEqual([
+      { path: "/en?", params: {} },
+      { path: "/en?/abc", params: {} },
+      { path: "/en?/abc/def", params: {} },
+    ]);
+    expect(pickPathsAndParams(nested, "/abc/def")).toEqual([
+      { path: "/en?", params: {} },
+      { path: "/en?/abc", params: {} },
+      { path: "/en?/abc/def", params: {} },
+    ]);
+  });
+
+  test("optional static segment in nested absolute routes (middle)", () => {
+    let nested = [
+      {
+        path: "/en",
+        children: [
+          {
+            path: "/en/abc?",
+            children: [
+              {
+                path: "/en/abc?/def",
+              },
+            ],
+          },
+        ],
+      },
+    ];
+
+    expect(pickPathsAndParams(nested, "/en/abc")).toEqual([
+      { path: "/en", params: {} },
+      { path: "/en/abc?", params: {} },
+    ]);
+    expect(pickPathsAndParams(nested, "/en/abc/def")).toEqual([
+      { path: "/en", params: {} },
+      { path: "/en/abc?", params: {} },
+      { path: "/en/abc?/def", params: {} },
+    ]);
+    expect(pickPathsAndParams(nested, "/en/def")).toEqual([
+      { path: "/en", params: {} },
+      { path: "/en/abc?", params: {} },
+      { path: "/en/abc?/def", params: {} },
+    ]);
+  });
 });
 
 describe("path matching with optional dynamic segments", () => {
diff --git a/packages/react-router/lib/router/utils.ts b/packages/react-router/lib/router/utils.ts
index 0ffdf25f16..80e783a4d2 100644
--- a/packages/react-router/lib/router/utils.ts
+++ b/packages/react-router/lib/router/utils.ts
@@ -982,10 +982,12 @@ function flattenRoutes<
   branches: RouteBranch<RouteObjectType>[] = [],
   parentsMeta: RouteMeta<RouteObjectType>[] = [],
   parentPath = "",
+  _hasParentOptionalSegments = false,
 ): RouteBranch<RouteObjectType>[] {
   let flattenRoute = (
     route: RouteObjectType,
     index: number,
+    hasParentOptionalSegments = _hasParentOptionalSegments,
     relativePath?: string,
   ) => {
     let meta: RouteMeta<RouteObjectType> = {
@@ -997,6 +999,17 @@ function flattenRoutes<
     };
 
     if (meta.relativePath.startsWith("/")) {
+      if (
+        !meta.relativePath.startsWith(parentPath) &&
+        hasParentOptionalSegments
+      ) {
+        // If we're inside of a parent route that has optional segments, we don't
+        // want to throw a hard error here because due to the route exploding
+        // approach, some of the routes won't match by design and we can just
+        // discard them instead.
+        // https://github.com/remix-run/react-router/issues/9925#issuecomment-1387252214
+        return;
+      }
       invariant(
         meta.relativePath.startsWith(parentPath),
         `Absolute route path "${meta.relativePath}" nested under path ` +
@@ -1021,7 +1034,13 @@ function flattenRoutes<
         `Index routes must not have child routes. Please remove ` +
           `all child routes from route path "${path}".`,
       );
-      flattenRoutes(route.children, branches, routesMeta, path);
+      flattenRoutes(
+        route.children,
+        branches,
+        routesMeta,
+        path,
+        hasParentOptionalSegments,
+      );
     }
 
     // Routes without a path shouldn't ever match by themselves unless they are
@@ -1042,7 +1061,7 @@ function flattenRoutes<
       flattenRoute(route, index);
     } else {
       for (let exploded of explodeOptionalSegments(route.path)) {
-        flattenRoute(route, index, exploded);
+        flattenRoute(route, index, true, exploded);
       }
     }
   });

From 36c9dee76999bbfbc86ddbb5da257a8a89ca0c6d Mon Sep 17 00:00:00 2001
From: Matt Brophy <matt@brophy.org>
Date: Wed, 6 Aug 2025 16:34:01 -0400
Subject: [PATCH 2/2] Add changeset

---
 .changeset/bright-bats-wave.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/bright-bats-wave.md

diff --git a/.changeset/bright-bats-wave.md b/.changeset/bright-bats-wave.md
new file mode 100644
index 0000000000..9ec9ffc905
--- /dev/null
+++ b/.changeset/bright-bats-wave.md
@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix usage of optional path segments in nested routes defined using absolute paths