From 863584924241e243475038dca0fb20664aad1af0 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Thu, 29 Aug 2024 15:37:04 -0700
Subject: [PATCH] Fix: Validating cross namespace  (#4304)

---
 ...sioning-cross-ns-ref-2024-7-29-18-19-44.md |  6 +++++
 packages/versioning/src/validate.ts           | 12 ++++++++-
 .../test/versioned-dependencies.test.ts       | 26 +++++++++++++++++++
 3 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 .chronus/changes/fix-versioning-cross-ns-ref-2024-7-29-18-19-44.md

diff --git a/.chronus/changes/fix-versioning-cross-ns-ref-2024-7-29-18-19-44.md b/.chronus/changes/fix-versioning-cross-ns-ref-2024-7-29-18-19-44.md
new file mode 100644
index 0000000000..65426bb013
--- /dev/null
+++ b/.chronus/changes/fix-versioning-cross-ns-ref-2024-7-29-18-19-44.md
@@ -0,0 +1,6 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/versioning"
+---
+
diff --git a/packages/versioning/src/validate.ts b/packages/versioning/src/validate.ts
index 5ce80dac3e..7114b13539 100644
--- a/packages/versioning/src/validate.ts
+++ b/packages/versioning/src/validate.ts
@@ -605,7 +605,17 @@ function validateTargetVersionCompatible(
 ) {
   const sourceAvailability = resolveAvailabilityForStack(program, source);
   const [sourceNamespace] = getVersions(program, sourceAvailability.type);
-
+  // If we cannot get source availability check if there is some different versioning across the stack which would mean we verify across namespace and is causing issues.
+  if (sourceAvailability.map === undefined) {
+    const sources = Array.isArray(source) ? source : [source];
+    const baseNs = getVersions(program, sources[0]);
+    for (const type of sources) {
+      const ns = getVersions(program, type);
+      if (ns !== baseNs) {
+        return undefined;
+      }
+    }
+  }
   const targetAvailability = resolveAvailabilityForStack(program, target);
   const [targetNamespace] = getVersions(program, targetAvailability.type);
   if (!targetAvailability.map || !targetNamespace) return;
diff --git a/packages/versioning/test/versioned-dependencies.test.ts b/packages/versioning/test/versioned-dependencies.test.ts
index cb1fa7ac14..e1f6cafd48 100644
--- a/packages/versioning/test/versioned-dependencies.test.ts
+++ b/packages/versioning/test/versioned-dependencies.test.ts
@@ -551,6 +551,32 @@ describe("versioning: dependencies", () => {
     runner = await createVersioningTestRunner();
   });
 
+  it("cross namespace operation", async () => {
+    const diagnostics = await runner.diagnose(`
+      @versioned(Versions)
+      namespace Service {
+        enum Versions {
+          v1,
+          v2,
+        }
+
+        @added(Versions.v2)
+        op test(purpose: FilePurpose): void;
+
+        @added(Versions.v2)
+        model FilePurpose {
+          a: string;
+        }
+      }
+
+      @useDependency(Service.Versions.v2)
+      namespace Client {
+        op testClient is Service.test;
+      }
+    `);
+    expectDiagnosticEmpty(diagnostics);
+  });
+
   it("use model defined in non versioned library spreading properties", async () => {
     const { MyService, Test } = (await runner.compile(`
       namespace NonVersionedLib {