Add more descriptive message for @pattern decorator when applied to union type

.chronus/changes/add-more-descriptive-message-for-union-2025-5-6-16-19-52.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Improved the error message for the `@pattern` decorator when applied to a `union` type. The new message is more descriptive and helps users understand how to correctly define string-compatible union types.

packages/compiler/src/lib/decorators.ts

@@ -294,13 +294,17 @@ function validateTargetingAString(
   target: Scalar | ModelProperty,
   decoratorName: string,
 ) {
-  const valid = isTypeIn(getPropertyType(target), (x) => isStringType(context.program, x));
+  const propertyType = getPropertyType(target);
+  const valid = isTypeIn(propertyType, (x) => isStringType(context.program, x));
   if (!valid) {
     reportDiagnostic(context.program, {
       code: "decorator-wrong-target",
       format: {
         decorator: decoratorName,
-        to: `type it is not a string`,
+        to:
+          propertyType.kind === "Union"
+            ? `a union type that is not string compatible. The union must explicitly include a string type, and all union values should be strings. For example: union Test { string, "A", "B" }`
+            : `type it is not a string`,
       },
       target: context.decoratorTarget,
     });

packages/compiler/test/decorators/decorators.test.ts

@@ -272,6 +272,34 @@ describe("compiler: built-in decorators", () => {
       });
     });
 
+    it("emit diagnostic if pattern targe is a non-string union", async () => {
+      const diagnostics = await runner.diagnose(`
+        model Employee {
+          @pattern("^[a-zA-Z0-9-]{3,24}$")
+          name: Test;
+        }
+        union Test { 1, int32 }
+      `);
+
+      expectDiagnostics(diagnostics, {
+        code: "decorator-wrong-target",
+        message:
+          'Cannot apply @pattern decorator to a union type that is not string compatible. The union must explicitly include a string type, and all union values should be strings. For example: union Test { string, "A", "B" }',
+      });
+    });
+
+    it("allows string union on @pattern", async () => {
+      const diagnostics = await runner.diagnose(`
+        model Employee {
+          @pattern("^[a-zA-Z0-9-]{3,24}$")
+          name: Test;
+        }
+        union Test { "A", "B", string }
+      `);
+
+      expectDiagnosticEmpty(diagnostics);
+    });
+
     it("emit diagnostic if pattern is not a valid RegEx", async () => {
       const diagnostics = await runner.diagnose(`
         model A {