Formatter: Fix valueof should keep parentheses in union and array expression

common/changes/@typespec/compiler/fix-formatter-keep-parens-valueof_2023-11-21-18-00.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/compiler",
+      "comment": "Formatter: Fix: `valueof` expression with parentheses around will preserve them when they are meaningful(For example inside a union or array expression)",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/compiler"
+}

packages/compiler/src/formatter/print/needs-parens.ts

@@ -16,6 +16,12 @@ export function needsParens(path: AstPath<Node>, options: TypeSpecPrettierOption
   // eslint-disable-next-line deprecation/deprecation
   const node = path.getValue();
   switch (node.kind) {
+    case SyntaxKind.ValueOfExpression:
+      return (
+        parent.kind === SyntaxKind.UnionExpression ||
+        parent.kind === SyntaxKind.ArrayExpression ||
+        parent.kind === SyntaxKind.IntersectionExpression
+      );
     case SyntaxKind.IntersectionExpression:
       return (
         parent.kind === SyntaxKind.UnionExpression || parent.kind === SyntaxKind.ArrayExpression

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

@@ -2429,6 +2429,41 @@ model Foo {
     });
   });
 
+  describe("valueof", () => {
+    it("format simple valueof", async () => {
+      await assertFormat({
+        code: `
+alias A =      valueof        string;
+`,
+        expected: `
+alias A = valueof string;
+`,
+      });
+    });
+
+    it("keeps parentheses around valueof inside a union", async () => {
+      await assertFormat({
+        code: `
+alias A =      (valueof        string) | Model;
+`,
+        expected: `
+alias A = (valueof string) | Model;
+`,
+      });
+    });
+
+    it("keeps parentheses around valueof inside a array expression", async () => {
+      await assertFormat({
+        code: `
+alias A =      (valueof        string)[];
+`,
+        expected: `
+alias A = (valueof string)[];
+`,
+      });
+    });
+  });
+
   describe("projections", () => {
     it("format projections", async () => {
       await assertFormat({