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

fix #4420

packages/type-utils/src/isTypeReadonly.ts

@@ -1,5 +1,6 @@
 import { ESLintUtils } from '@typescript-eslint/experimental-utils';
 import {
+  isConditionalType,
   isObjectType,
   isUnionType,
   isUnionOrIntersectionType,
@@ -198,6 +199,20 @@ 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)) {