Fix TCGC crash when a merged service omits a version for a depended library

.chronus/changes/fix-tcgc-multiple-service-dep-undefined-2026-05-27-03-38-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Fix "Cannot read properties of undefined" crash in the `inconsistent-multiple-service-dependency` validation when a service merged into a multi-service client does not specify a version for a depended library (e.g. its latest service version has no matching `@useDependency` entry). The validation now falls back to the latest version of the depended library, matching the behavior of downstream emitters.

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

@@ -33,7 +33,7 @@ import {
   isBody,
   isBodyRoot,
 } from "@typespec/http";
-import { resolveVersions } from "@typespec/versioning";
+import { getVersion, resolveVersions, type Version } from "@typespec/versioning";
 import {
   AccessDecorator,
   AlternateTypeDecorator,
@@ -274,8 +274,21 @@ function validateMultipleServiceDependencyVersions(
     for (const [depNs, depVersion] of resolutions[resolutions.length - 1].versions) {
       // Ignore versions of the merged services themselves.
       if (serviceSet.has(depNs)) continue;
+      // When the service does not specify a version for the depended library
+      // (e.g. the latest service version has no `@useDependency` mapping for it),
+      // fall back to the latest version of the depended library, matching the
+      // behavior expected by downstream emitters.
+      let resolvedDepVersion: Version | undefined = depVersion;
+      if (resolvedDepVersion === undefined) {
+        const depVersionMap = getVersion(program, depNs);
+        const allDepVersions = depVersionMap?.getVersions();
+        if (allDepVersions && allDepVersions.length > 0) {
+          resolvedDepVersion = allDepVersions[allDepVersions.length - 1];
+        }
+      }
+      if (resolvedDepVersion === undefined) continue;
       const versions = depVersions.get(depNs) ?? new Set<string>();
-      versions.add(depVersion.value ?? depVersion.name);
+      versions.add(resolvedDepVersion.value ?? resolvedDepVersion.name);
       depVersions.set(depNs, versions);
     }
   }

packages/typespec-client-generator-core/test/clients/structure.test.ts

@@ -2196,6 +2196,63 @@ it("no warning when multiple services share the same dependency version", async
   expectDiagnostics(diagnostics, []);
 });
 
+it("no crash when a merged service does not specify a version for the depended library", async () => {
+  const [{ program }, diagnostics] = await SimpleBaseTester.compileAndDiagnose(
+    createClientCustomizationInput(
+      `
+        @versioned(LibVersions)
+        namespace SharedLib {
+          enum LibVersions {
+            v1: "v1",
+            v2: "v2",
+          }
+        }
+
+        @service
+        @versioned(VersionsA)
+        namespace ServiceA {
+          enum VersionsA {
+            @useDependency(SharedLib.LibVersions.v2)
+            av1,
+          }
+          op a(): void;
+        }
+        @service
+        @versioned(VersionsB)
+        namespace ServiceB {
+          enum VersionsB {
+            @useDependency(SharedLib.LibVersions.v2)
+            bv1,
+            bv2,
+          }
+          op b(): void;
+        }`,
+      `
+        @client(
+          {
+            name: "CombineClient",
+            service: [ServiceA, ServiceB],
+            autoMergeService: true,
+          }
+        )
+        namespace CombineClient {}
+      `,
+    ),
+  );
+  await createSdkContextForTester(program);
+  // ServiceA explicitly uses SharedLib.v2; ServiceB does not specify a version
+  // so the validation falls back to SharedLib's latest version (v2). Both
+  // resolve to the same version, so no inconsistency warning is reported.
+  expectDiagnostics(
+    diagnostics.filter(
+      (d) =>
+        d.code ===
+        "@azure-tools/typespec-client-generator-core/inconsistent-multiple-service-dependency",
+    ),
+    [],
+  );
+});
+
 it("multiple clients from single service", async () => {
   const { program } = await SimpleBaseTester.compile(
     createClientCustomizationInput(