Fix regression using dep (#4157)

Previous PR https://github.com/microsoft/typespec/pull/4145/files fix an
issue with referencing items from another sub namespace and simplified
the logic but seems like this is making the
typespec-azure-resource-manager library error.

The way the library was written kinda makes it seems like there is
indeed an error but can't change that right now so just reverting as we
plan to get rid of this error anyway

.chronus/changes/fix-regression-using-dep-2024-7-13-2-12-20.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: internal
+packages:
+  - "@typespec/versioning"
+---
+
+Fix regression using dep

packages/versioning/src/validate.ts

@@ -376,6 +376,7 @@ function validateVersionedNamespaceUsage(
       const sourceVersionedNamespace = source && findVersionedNamespace(program, source);
       if (
         targetVersionedNamespace !== undefined &&
+        !(source && (isSubNamespace(target, source) || isSubNamespace(source, target))) &&
         sourceVersionedNamespace !== targetVersionedNamespace &&
         dependencies?.get(targetVersionedNamespace) === undefined
       ) {
@@ -440,6 +441,19 @@ function validateVersionedPropertyNames(program: Program, source: Type) {
   }
 }
 
+function isSubNamespace(parent: Namespace, child: Namespace): boolean {
+  let current: Namespace | undefined = child;
+
+  while (current && current.name !== "") {
+    if (current === parent) {
+      return true;
+    }
+    current = current.namespace;
+  }
+
+  return false;
+}
+
 function validateMadeOptional(program: Program, target: Type) {
   if (target.kind === "ModelProperty") {
     const madeOptionalOn = getMadeOptionalOn(program, target);

packages/versioning/test/versioned-dependencies.test.ts

@@ -449,6 +449,24 @@ describe("versioning: reference versioned library", () => {
       expectDiagnosticEmpty(diagnostics);
     });
 
+    // LEGACY test due to arm depending on it. Should remove when we relax using-versioned-library rule
+    it("doesn't emit diagnostic when parent namespace reference sub namespace that is versioned differently", async () => {
+      const diagnostics = await runner.diagnose(`
+        @versioned(Versions)
+        namespace MyService {
+          enum Versions {m1}
+          model Foo is SubNamespace.Bar;
+          
+          @versioned(SubNamespace.Versions)
+          namespace SubNamespace {
+            enum Versions { s1 }
+            model Bar {}
+          }
+        }
+    `);
+      expectDiagnosticEmpty(diagnostics);
+    });
+
     it("succeed if sub namespace of versioned service reference versioned library", async () => {
       const diagnostics = await runner.diagnose(`
         @versioned(Versions)