Fix: Error out when using properties in array model (#2873)

fix #2758

---------

Co-authored-by: Mark Cowlishaw <markcowl@microsoft.com>

.changeset/lucky-colts-hang.md

@@ -0,0 +1,5 @@
+---
+"@typespec/compiler": patch
+---
+
+Fix: Error out when using properties in array model

packages/compiler/src/core/checker.ts

@@ -2829,6 +2829,15 @@ export function createChecker(program: Program): Checker {
     if (indexer === undefined) {
       return;
     }
+    if (indexer.key.name === "integer") {
+      reportCheckerDiagnostics([
+        createDiagnostic({
+          code: "no-array-properties",
+          target: diagnosticTarget,
+        }),
+      ]);
+      return;
+    }
 
     const [valid, diagnostics] = isTypeAssignableTo(
       property.type,

packages/compiler/src/core/messages.ts

@@ -298,6 +298,12 @@ const diagnostics = {
       array: "Cannot intersect an array model.",
     },
   },
+  "no-array-properties": {
+    severity: "error",
+    messages: {
+      default: "Array models cannot have any properties.",
+    },
+  },
   "intersect-duplicate-property": {
     severity: "error",
     messages: {

packages/compiler/test/checker/model.test.ts

@@ -663,6 +663,38 @@ describe("compiler: models", () => {
       strictEqual(A.indexer.value.kind, "Union");
     });
 
+    it("model is array cannot have properties", async () => {
+      testHost.addTypeSpecFile(
+        "main.tsp",
+        `
+        @test model A is string[] {
+          prop: string;
+        }
+        `
+      );
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, {
+        code: "no-array-properties",
+        message: "Array models cannot have any properties.",
+      });
+    });
+
+    it("model extends array cannot have properties", async () => {
+      testHost.addTypeSpecFile(
+        "main.tsp",
+        `
+        @test model A extends Array<string> {
+          prop: string;
+        }
+        `
+      );
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, {
+        code: "no-array-properties",
+        message: "Array models cannot have any properties.",
+      });
+    });
+
     it("doesn't allow duplicate properties", async () => {
       testHost.addTypeSpecFile(
         "main.tsp",