fix(type-utils): union types always being marked as readonly (#4419)

Author: RebeccaStevens

Diff for: packages/type-utils/src/isTypeReadonly.ts

@@ -216,7 +216,8 @@ function isTypeReadonlyRecurser(
     const result = unionTypeParts(type).every(
       t =>
         seenTypes.has(t) ||
-        isTypeReadonlyRecurser(checker, t, options, seenTypes),
+        isTypeReadonlyRecurser(checker, t, options, seenTypes) ===
+          Readonlyness.Readonly,
     );
     const readonlyness = result ? Readonlyness.Readonly : Readonlyness.Mutable;
     return readonlyness;

Diff for: packages/type-utils/tests/isTypeReadonly.test.ts

@@ -137,6 +137,33 @@ describe('isTypeReadonly', () => {
           );
         });
       });
+
+      describe('Union', () => {
+        describe('is readonly', () => {
+          const runTests = runTestIsReadonly;
+
+          it.each([
+            [
+              'type Test = Readonly<{ foo: string; bar: number; }> & Readonly<{ bar: number; }>;',
+            ],
+            ['type Test = readonly string[] | readonly number[];'],
+          ])('handles a union of 2 fully readonly types', runTests);
+        });
+
+        describe('is not readonly', () => {
+          const runTests = runTestIsNotReadonly;
+
+          it.each([
+            ['type Test = { foo: string; bar: number; } | { bar: number; };'],
+            [
+              'type Test = { foo: string; bar: number; } | Readonly<{ bar: number; }>;',
+            ],
+            [
+              'type Test = Readonly<{ foo: string; bar: number; }> | { bar: number; };',
+            ],
+          ])('handles a union of non fully readonly types', runTests);
+        });
+      });
     });
 
     describe('treatMethodsAsReadonly', () => {