Fix expression defined in alias doesn't have namespace assigned (#4146)

fix [#3438](https://github.com/microsoft/typespec/issues/3438)

.chronus/changes/fix-alias-expr-2024-7-12-19-19-19.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix model expression defined in alias will resolve its namespace from the namespace where the alias was declared

packages/compiler/src/core/checker.ts

@@ -2409,6 +2409,7 @@ export function createChecker(program: Program): Checker {
 
   function getParentNamespaceType(
     node:
+      | AliasStatementNode
       | ModelStatementNode
       | ScalarStatementNode
       | NamespaceStatementNode
@@ -2432,6 +2433,7 @@ export function createChecker(program: Program): Checker {
       let parent: Node | undefined = node.parent;
       while (parent !== undefined) {
         if (
+          parent.kind === SyntaxKind.AliasStatement ||
           parent.kind === SyntaxKind.ModelStatement ||
           parent.kind === SyntaxKind.ScalarStatement ||
           parent.kind === SyntaxKind.OperationStatement ||

packages/compiler/test/checker/alias.test.ts

@@ -1,6 +1,6 @@
 import { ok, strictEqual } from "assert";
 import { beforeEach, describe, it } from "vitest";
-import { Model, Type, Union } from "../../src/core/types.js";
+import { Model, Namespace, Type, Union } from "../../src/core/types.js";
 import {
   TestHost,
   createTestHost,
@@ -194,6 +194,28 @@ describe("compiler: aliases", () => {
     strictEqual(Baz.properties.get("x")!.type, Bar);
   });
 
+  it("model expression defined in alias use containing namespace", async () => {
+    testHost.addTypeSpecFile(
+      "main.tsp",
+      `
+      @test namespace Foo {
+        alias B = {a: string};
+      }
+      @test model Test {
+        prop: Foo.B;
+      }
+      `
+    );
+
+    const { Test, Foo } = (await testHost.compile("./")) as {
+      Foo: Namespace;
+      Test: Model;
+    };
+
+    const expr = Test.properties.get("prop")!.type as Model;
+    strictEqual(expr.namespace, Foo);
+  });
+
   it("emit diagnostics if assign itself", async () => {
     testHost.addTypeSpecFile(
       "main.tsp",