Support serialization of object examples on unions (#5180)

This PR fixes this issue:
* #4869

Currently, the `@typespec/openapi3` emitter supports `@example`s for
`model`s, but outputs incorrect examples for `union`s.

For example ([playground
link](https://typespec.io/playground/?c=QGV4YW1wbGUoI3sgYTogMSB9KQptb2RlbCBBIHsKIMQUaW50ZWdlcjsKfQoK1C91bmlvbiBVxS9udW1iZXI6xQzMPSwKICB9xAV0ZXh0yB9zOiBzdHJpbmfHHn0K&e=%40typespec%2Fopenapi3&options=%7B%7D)):
```typespec
@example(#{ a: 1 })
union U {
  number: {
    a: integer,
  },
  text: {
    s: string,
  },
}
```

should output with `{ a: 1 }` as the example, but instead it is `{}`.

The issue manifests in the `serializeValueAsJson` function in
packages/compiler/src/lib/examples.ts, and occurs because the
`resolveUnions` function in the same file fails to detect the
appropriate union variant.

With this PR, the example is emitted correctly. I've added a unit test
to confirm this.

---------

Co-authored-by: Christopher Radek <[email protected]>

Author: RichardCPoint

.chronus/changes/rejc2-support-serialization-of-object-examples-on-unions-2024-11-6-14-7-58.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fixed serialization of object examples on unions

packages/compiler/src/lib/examples.ts

@@ -72,11 +72,12 @@ function resolveUnions(program: Program, value: ObjectValue, type: Type): Type |
   if (type.kind !== "Union") {
     return type;
   }
+  const exactValueType = program.checker.getValueExactType(value);
   for (const variant of type.variants.values()) {
     if (
       ignoreDiagnostics(
         program.checker.isTypeAssignableTo(
-          value.type.projectionBase ?? value.type,
+          exactValueType ?? value.type.projectionBase ?? value.type,
           variant.type.projectionBase ?? variant.type,
           value,
         ),

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

@@ -173,7 +173,7 @@ describe("@example", () => {
   });
 
   describe("union", () => {
-    it("valid", async () => {
+    it("valid for union member reference", async () => {
       const { program, examples, target } = await getExamplesFor(`
       @example(test.a)
       @test union test {a: "a", b: "b"}
@@ -182,6 +182,30 @@ describe("@example", () => {
       expect(serializeValueAsJson(program, examples[0].value, target)).toEqual("a");
     });
 
+    it("valid for object value", async () => {
+      const { program, examples, target } = await getExamplesFor(`
+        model A {
+          type: "a";
+          a: string;
+        }
+        model B {
+          type: "b";
+          b: numeric;
+        }
+
+        @example(#{
+          type: "a",
+          a: "a string",
+        })
+        @test union test {a: A, b: B}
+      `);
+      expect(examples).toHaveLength(1);
+      expect(serializeValueAsJson(program, examples[0].value, target)).toEqual({
+        type: "a",
+        a: "a string",
+      });
+    });
+
     it("emit diagnostic for unassignable value", async () => {
       const diagnostics = await diagnoseCode(`
         @example(1)