[tcgc] fix wrong unexpected-pageable-operation-return-type because …

…of nullable type (#1935)

when a method has several responses and only one of them has pageable
body. tcgc's method return type will be a wrapped nullable type which
real type is that response with body. so, when trying to find property,
we need to peel off the wrapped nullable.

.chronus/changes/fix_unbranded_page_issue-2024-10-29-17-58-58.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+fix wrong `unexpected-pageable-operation-return-type` because of nullable type

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

@@ -140,11 +140,17 @@ function getSdkPagingServiceMethod<TServiceOperation extends SdkServiceOperation
     getSdkBasicServiceMethod<TServiceOperation>(context, operation, client),
   );
 
+  // nullable response type means the underlaying operation has multiple responses and only one of them is not empty, which is what we want
+  let responseType = basic.response.type;
+  if (responseType?.kind === "nullable") {
+    responseType = responseType.type;
+  }
+
   // normal paging
   if (isList(context.program, operation)) {
     const pagingOperation = diagnostics.pipe(getPagingOperation(context.program, operation));
 
-    if (basic.response.type?.__raw?.kind !== "Model" || !pagingOperation) {
+    if (responseType?.__raw?.kind !== "Model" || !pagingOperation) {
       diagnostics.add(
         createDiagnostic({
           code: "unexpected-pageable-operation-return-type",
@@ -163,18 +169,18 @@ function getSdkPagingServiceMethod<TServiceOperation extends SdkServiceOperation
 
     basic.response.resultPath = getPropertyPathFromModel(
       context,
-      basic.response.type?.__raw,
+      responseType?.__raw,
       (p) => p === pagingOperation.output.pageItems.property,
     );
     const nextLinkPath = pagingOperation.output.nextLink
       ? getPropertyPathFromModel(
           context,
-          basic.response.type?.__raw,
+          responseType?.__raw,
           (p) => p === pagingOperation.output.nextLink!.property,
         )
       : undefined;
 
-    context.__pagedResultSet.add(basic.response.type);
+    context.__pagedResultSet.add(responseType);
     // tcgc will let all paging method return a list of items
     basic.response.type = diagnostics.pipe(
       getClientTypeWithDiagnostics(context, pagingOperation?.output.pageItems.property.type),
@@ -190,7 +196,7 @@ function getSdkPagingServiceMethod<TServiceOperation extends SdkServiceOperation
   // azure core paging
   const pagedMetadata = getPagedResult(context.program, operation)!;
 
-  if (basic.response.type?.__raw?.kind !== "Model" || !pagedMetadata.itemsProperty) {
+  if (responseType?.__raw?.kind !== "Model" || !pagedMetadata.itemsProperty) {
     diagnostics.add(
       createDiagnostic({
         code: "unexpected-pageable-operation-return-type",
@@ -207,7 +213,7 @@ function getSdkPagingServiceMethod<TServiceOperation extends SdkServiceOperation
     });
   }
 
-  context.__pagedResultSet.add(basic.response.type);
+  context.__pagedResultSet.add(responseType);
 
   // tcgc will let all paging method return a list of items
   basic.response.type = diagnostics.pipe(

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

@@ -69,6 +69,31 @@ describe("typespec-client-generator-core: paged operation", () => {
     strictEqual(response.resultPath, "tests");
   });
 
+  it("nullable paged result", async () => {
+    await runner.compileWithBuiltInService(`
+      @list
+      op test(): ListTestResult | NotFoundResponse;
+      model ListTestResult {
+        @pageItems
+        tests: Test[];
+        @TypeSpec.nextLink
+        next: string;
+      }
+      model Test {
+        id: string;
+      }
+    `);
+    const sdkPackage = runner.context.sdkPackage;
+    const method = getServiceMethodOfClient(sdkPackage);
+    strictEqual(method.name, "test");
+    strictEqual(method.kind, "paging");
+    strictEqual(method.nextLinkPath, "next");
+
+    const response = method.response;
+    strictEqual(response.kind, "method");
+    strictEqual(response.resultPath, "tests");
+  });
+
   it("normal paged result with encoded name", async () => {
     await runner.compileWithBuiltInService(`
       @list