fix(type-utils): isTypeReadonly now handles conditional types

packages/type-utils/src/isTypeReadonly.ts

@@ -1,5 +1,6 @@
 import { ESLintUtils } from '@typescript-eslint/utils';
 import {
+  isConditionalType,
   isObjectType,
   isUnionType,
   isUnionOrIntersectionType,
@@ -223,6 +224,20 @@ function isTypeReadonlyRecurser(
     return readonlyness;
   }
 
+  if (isConditionalType(type)) {
+    const result = [type.root.node.trueType, type.root.node.falseType]
+      .map(checker.getTypeFromTypeNode)
+      .every(
+        t =>
+          seenTypes.has(t) ||
+          isTypeReadonlyRecurser(checker, t, options, seenTypes) ===
+            Readonlyness.Readonly,
+      );
+
+    const readonlyness = result ? Readonlyness.Readonly : Readonlyness.Mutable;
+    return readonlyness;
+  }
+
   // all non-object, non-intersection types are readonly.
   // this should only be primitive types
   if (!isObjectType(type) && !isUnionOrIntersectionType(type)) {

packages/type-utils/tests/isTypeReadonly.test.ts

@@ -164,6 +164,38 @@ describe('isTypeReadonly', () => {
           ])('handles a union of non fully readonly types', runTests);
         });
       });
+
+      describe('Conditional Types', () => {
+        describe('is readonly', () => {
+          const runTests = runTestIsReadonly;
+
+          it.each([
+            [
+              'type Test<T> = T extends readonly number[] ? readonly string[] : readonly number[];',
+            ],
+          ])('handles conditional type that are fully readonly', runTests);
+
+          it.each([
+            [
+              'type Test<T> = T extends number[] ? readonly string[] : readonly number[];',
+            ],
+          ])('should ignore mutable conditions', runTests);
+        });
+
+        describe('is not readonly', () => {
+          const runTests = runTestIsNotReadonly;
+
+          it.each([
+            ['type Test<T> = T extends number[] ? string[] : number[];'],
+            [
+              'type Test<T> = T extends number[] ? string[] : readonly number[];',
+            ],
+            [
+              'type Test<T> = T extends number[] ? readonly string[] : number[];',
+            ],
+          ])('handles non fully readonly conditional types', runTests);
+        });
+      });
     });
 
     describe('treatMethodsAsReadonly', () => {