[TCGC] Consider inheritance when mapping first segment for paging result.

.chronus/changes/tcgc-fix_paging_metadata-2025-7-14-17-14-29.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Consider inheritance when mapping first segment for paging result.

packages/typespec-client-generator-core/src/methods.ts

@@ -354,13 +354,17 @@ function mapFirstSegmentForResultSegments(
   if (resultSegments.length > 0 && responseModel) {
     for (let i = 0; i < resultSegments.length; i++) {
       const segment = resultSegments[i];
-      for (const property of responseModel.properties ?? []) {
-        if (
-          property.__raw &&
-          findRootSourceProperty(property.__raw) === findRootSourceProperty(segment)
-        ) {
-          return [property.__raw, ...resultSegments.slice(i + 1)];
+      let current: SdkModelType | undefined = responseModel;
+      while (current) {
+        for (const property of current.properties ?? []) {
+          if (
+            property.__raw &&
+            findRootSourceProperty(property.__raw) === findRootSourceProperty(segment)
+          ) {
+            return [property.__raw, ...resultSegments.slice(i + 1)];
+          }
         }
+        current = current.baseModel;
       }
     }
   }

packages/typespec-client-generator-core/test/methods/paged-operation.test.ts

@@ -1101,3 +1101,47 @@ it("paged result with body root", async () => {
   strictEqual(response.resultSegments[0], sdkPackage.models[0].properties[0]);
   strictEqual(method.pagingMetadata.pageItemsSegments, response.resultSegments);
 });
+
+it("next link with body root and inheritance", async () => {
+  await runner.compileWithBuiltInService(`
+    @list
+    op test(): TestResponse<ListTestResult>;
+
+    model ListMeta{
+      @nextLink
+      nextLink: url;
+    }
+
+    model ListTestResult extends ListMeta {
+      @pageItems
+      tests: Test[];
+
+      @header
+      h: string;
+    }
+    model Test {
+      id: string;
+    }
+    model TestResponse<ResponseBody> {
+      ...OkResponse;
+
+      @bodyRoot
+      body: ResponseBody;
+    }
+  `);
+  const sdkPackage = runner.context.sdkPackage;
+  const method = getServiceMethodOfClient(sdkPackage);
+  strictEqual(method.name, "test");
+  strictEqual(method.kind, "paging");
+  strictEqual(method.pagingMetadata.nextLinkSegments?.length, 1);
+  strictEqual(
+    method.pagingMetadata.nextLinkSegments[0],
+    sdkPackage.models[0].baseModel?.properties[0],
+  );
+
+  const response = method.response;
+  strictEqual(response.kind, "method");
+  strictEqual(response.resultSegments?.length, 1);
+  strictEqual(response.resultSegments[0], sdkPackage.models[0].properties[0]);
+  strictEqual(method.pagingMetadata.pageItemsSegments, response.resultSegments);
+});