Validation preventing duplicate version values

common/changes/@typespec/versioning/versioning-DuplicateValueError_2023-05-03-22-25.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/versioning",
+      "comment": "Added validation preventing version enums from having duplicate values.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/versioning"
+}

packages/versioning/src/lib.ts

@@ -39,6 +39,12 @@ const libDef = {
         default: paramMessage`The provided version '${"version"}' from '${"enumName"}' is not declared as a version enum. Use '@versioned(${"enumName"})' on the containing namespace.`,
       },
     },
+    "version-duplicate": {
+      severity: "error",
+      messages: {
+        default: paramMessage`Multiple versions from '${"name"}' resolve to the same value. Version enums must resolve to unique values.`,
+      },
+    },
     "using-versioned-library": {
       severity: "error",
       messages: {

packages/versioning/src/validate.ts

@@ -95,6 +95,18 @@ export function $onValidate(program: Program) {
       },
       namespace: (namespace) => {
         const [_, versionMap] = getVersions(program, namespace);
+        const size = versionMap?.size;
+        const values = new Set(versionMap?.getVersions().map((v) => v.value));
+        if (size && values) {
+          if (size !== values.size) {
+            const enumName = versionMap.getVersions()[0].enumMember.enum.name;
+            reportDiagnostic(program, {
+              code: "version-duplicate",
+              format: { name: enumName },
+              target: namespace,
+            });
+          }
+        }
         const serviceProps = getService(program, namespace);
         if (serviceProps?.version !== undefined && versionMap !== undefined) {
           reportDiagnostic(program, {

packages/versioning/test/incompatible-versioning.test.ts

@@ -42,6 +42,25 @@ describe("versioning: incompatible use of decorators", () => {
       severity: "error",
     });
   });
+
+  it("emit diagnostic when version enum has duplicate values", async () => {
+    const diagnostics = await runner.diagnose(`
+    @versioned(Versions)
+    namespace DemoService;
+
+    enum Versions {
+      v1: "v1",
+      v2: "v2",
+      latest: "v2",
+    }
+    `);
+    expectDiagnostics(diagnostics, {
+      code: "@typespec/versioning/version-duplicate",
+      message:
+        "Multiple versions from 'Versions' resolve to the same value. Version enums must resolve to unique values.",
+      severity: "error",
+    });
+  });
 });
 
 describe("versioning: validate incompatible references", () => {