Formatting a union should not create a 1-element union type node (#27582)

Author: weswigham

src/compiler/checker.ts

@@ -3307,6 +3307,9 @@ namespace ts {
                 }
                 if (type.flags & (TypeFlags.Union | TypeFlags.Intersection)) {
                     const types = type.flags & TypeFlags.Union ? formatUnionTypes((<UnionType>type).types) : (<IntersectionType>type).types;
+                    if (length(types) === 1) {
+                        return typeToTypeNodeHelper(types[0], context);
+                    }
                     const typeNodes = mapToTypeNodes(types, context, /*isBareList*/ true);
                     if (typeNodes && typeNodes.length > 0) {
                         const unionOrIntersectionTypeNode = createUnionOrIntersectionTypeNode(type.flags & TypeFlags.Union ? SyntaxKind.UnionType : SyntaxKind.IntersectionType, typeNodes);

tests/baselines/reference/declarationEmitNoNonRequiredParens.js

@@ -0,0 +1,29 @@
+//// [declarationEmitNoNonRequiredParens.ts]
+export enum Test {
+    A, B, C
+}
+
+export type TestType = typeof Test;
+
+export const bar = (null as TestType[Extract<keyof TestType, string>][]);
+
+//// [declarationEmitNoNonRequiredParens.js]
+"use strict";
+exports.__esModule = true;
+var Test;
+(function (Test) {
+    Test[Test["A"] = 0] = "A";
+    Test[Test["B"] = 1] = "B";
+    Test[Test["C"] = 2] = "C";
+})(Test = exports.Test || (exports.Test = {}));
+exports.bar = null;
+
+
+//// [declarationEmitNoNonRequiredParens.d.ts]
+export declare enum Test {
+    A = 0,
+    B = 1,
+    C = 2
+}
+export declare type TestType = typeof Test;
+export declare const bar: Test[];

tests/baselines/reference/declarationEmitNoNonRequiredParens.symbols

@@ -0,0 +1,20 @@
+=== tests/cases/compiler/declarationEmitNoNonRequiredParens.ts ===
+export enum Test {
+>Test : Symbol(Test, Decl(declarationEmitNoNonRequiredParens.ts, 0, 0))
+
+    A, B, C
+>A : Symbol(Test.A, Decl(declarationEmitNoNonRequiredParens.ts, 0, 18))
+>B : Symbol(Test.B, Decl(declarationEmitNoNonRequiredParens.ts, 1, 6))
+>C : Symbol(Test.C, Decl(declarationEmitNoNonRequiredParens.ts, 1, 9))
+}
+
+export type TestType = typeof Test;
+>TestType : Symbol(TestType, Decl(declarationEmitNoNonRequiredParens.ts, 2, 1))
+>Test : Symbol(Test, Decl(declarationEmitNoNonRequiredParens.ts, 0, 0))
+
+export const bar = (null as TestType[Extract<keyof TestType, string>][]);
+>bar : Symbol(bar, Decl(declarationEmitNoNonRequiredParens.ts, 6, 12))
+>TestType : Symbol(TestType, Decl(declarationEmitNoNonRequiredParens.ts, 2, 1))
+>Extract : Symbol(Extract, Decl(lib.es5.d.ts, --, --))
+>TestType : Symbol(TestType, Decl(declarationEmitNoNonRequiredParens.ts, 2, 1))
+

tests/baselines/reference/declarationEmitNoNonRequiredParens.types

@@ -0,0 +1,20 @@
+=== tests/cases/compiler/declarationEmitNoNonRequiredParens.ts ===
+export enum Test {
+>Test : Test
+
+    A, B, C
+>A : Test.A
+>B : Test.B
+>C : Test.C
+}
+
+export type TestType = typeof Test;
+>TestType : typeof Test
+>Test : typeof Test
+
+export const bar = (null as TestType[Extract<keyof TestType, string>][]);
+>bar : Test[]
+>(null as TestType[Extract<keyof TestType, string>][]) : Test[]
+>null as TestType[Extract<keyof TestType, string>][] : Test[]
+>null : null
+

tests/cases/compiler/declarationEmitNoNonRequiredParens.ts

@@ -0,0 +1,8 @@
+// @declaration: true
+export enum Test {
+    A, B, C
+}
+
+export type TestType = typeof Test;
+
+export const bar = (null as TestType[Extract<keyof TestType, string>][]);