Fix formatting of union

common/changes/@typespec/compiler/fix-format-union_2023-09-19-19-07.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/compiler",
+      "comment": "Fix: Union with anonymous variant were formatted with an extra leading `:`",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/compiler"
+}

packages/compiler/src/formatter/print/printer.ts

@@ -766,8 +766,8 @@ export function printUnionVariant(
   options: TypeSpecPrettierOptions,
   print: PrettierChildPrint
 ) {
-  const id = path.call(print, "id");
-  const value = [": ", path.call(print, "value")];
+  const id = path.node.id === undefined ? "" : [path.call(print, "id"), ": "];
+  const value = [id, path.call(print, "value")];
   const { decorators } = printDecorators(path, options, print, {
     tryInline: DecoratorsTryInline.unionVariant,
   });

packages/compiler/test/formatter/formatter.test.ts

@@ -1558,6 +1558,93 @@ enum Foo {
         ...       Bar  , One: "1",
         ...Baz  ; Two: "2",
 
+}
+`,
+        expected: `
+enum Foo {
+  ...Bar,
+  One: "1",
+  ...Baz,
+  Two: "2",
+}
+`,
+      });
+    });
+  });
+
+  describe("union", () => {
+    it("format simple union", async () => {
+      await assertFormat({
+        code: `
+union      Foo       {    A,        B}
+union      Bar       
+      {    A,    
+            B}
+`,
+        expected: `
+union Foo {
+  A,
+  B,
+}
+union Bar {
+  A,
+  B,
+}
+`,
+      });
+    });
+
+    it("format named union", async () => {
+      await assertFormat({
+        code: `
+        union      Foo       {    a: A,        b:       B}
+`,
+        expected: `
+enum Foo {
+  a: A,
+  a: B,
+}
+
+`,
+      });
+    });
+
+    it("separate members if there is decorators", async () => {
+      await assertFormat({
+        code: `
+union      Foo       {   
+  @doc("foo") 
+        a: A,    @doc("bar") 
+           b    : B, 
+
+
+
+      @doc("third")   
+       c    : C}
+
+`,
+        expected: `
+enum Foo {
+  @doc("foo")
+  a: A,
+
+  @doc("bar")
+  b: B,
+
+  @doc("third")
+  c: C,
+}
+`,
+      });
+    });
+
+    it("format spread members", async () => {
+      await assertFormat({
+        code: `
+enum Foo {
+        ...       Bar  , One: "1",
+        ...Baz  ; Two: "2",
+
 }
 `,
         expected: `