fix: support union type in React react-only props rule (#477)

This change fixes the following (simplified) example from our application:

```
type Props =
  | {
      +children: React$Node,
      +title: React$Node,
      +withHiddenTitle?: false,
    }
  | {
      +children: React$Node,
      +title?: React$Node,
      +withHiddenTitle: true,
    };

const props: Props = {children:"", withHiddenTitle: true}
props.title = ""; // correct Flow error
```

Flow correctly throws the following error:

```
14: props.title = "";
          ^ Cannot assign empty string to `props.title` because property `title` is not writable. [cannot-write]
```

However, `flowtype/require-readonly-react-props` Eslint rule throws the following (incorrect) error:

```
Props must be $ReadOnly
```

For simplification, all of the objects in the union must be readonly even though it's probably not a strict requirement in Flow. This can be improved later when needed.

.README/rules/require-readonly-react-props.md

@@ -1,6 +1,6 @@
 ### `require-readonly-react-props`
 
-This rule validates that React props are marked as $ReadOnly. React props are immutable and modifying them could lead to unexpected results. Marking prop shapes as $ReadOnly avoids these issues.
+This rule validates that React props are marked as `$ReadOnly`. React props are immutable and modifying them could lead to unexpected results. Marking prop shapes as `$ReadOnly` avoids these issues.
 
 The rule tries its best to work with both class and functional components. For class components, it does a fuzzy check for one of "Component", "PureComponent", "React.Component" and "React.PureComponent". It doesn't actually infer that those identifiers resolve to a proper `React.Component` object.
 

README.md

@@ -3451,7 +3451,7 @@ const f: fn = (a, b) => {}
 <a name="eslint-plugin-flowtype-rules-require-readonly-react-props"></a>
 ### <code>require-readonly-react-props</code>
 
-This rule validates that React props are marked as $ReadOnly. React props are immutable and modifying them could lead to unexpected results. Marking prop shapes as $ReadOnly avoids these issues.
+This rule validates that React props are marked as `$ReadOnly`. React props are immutable and modifying them could lead to unexpected results. Marking prop shapes as `$ReadOnly` avoids these issues.
 
 The rule tries its best to work with both class and functional components. For class components, it does a fuzzy check for one of "Component", "PureComponent", "React.Component" and "React.PureComponent". It doesn't actually infer that those identifiers resolve to a proper `React.Component` object.
 
@@ -3574,6 +3574,13 @@ export type Props = {}; class Foo extends Component<Props> { }
 type Props = {| foo: string |}; class Foo extends Component<Props> { }
 // Message: Props must be $ReadOnly
 
+type Props = {| foo: string |} | {| bar: number |}; class Foo extends Component<Props> { }
+// Message: Props must be $ReadOnly
+
+// Options: [{"useImplicitExactTypes":true}]
+type Props = { foo: string } | { bar: number }; class Foo extends Component<Props> { }
+// Message: Props must be $ReadOnly
+
 type Props = {| +foo: string, ...bar |}; class Foo extends Component<Props> { }
 // Message: Props must be $ReadOnly
 
@@ -3622,6 +3629,11 @@ type Props = {| +foo: string |}; class Foo extends Component<Props> { }
 
 type Props = {| +foo: string, +bar: number |}; class Foo extends Component<Props> { }
 
+type Props = {| +foo: string |} | {| +bar: number |}; class Foo extends Component<Props> { }
+
+// Options: [{"useImplicitExactTypes":true}]
+type Props = { +foo: string } | { +bar: number }; class Foo extends Component<Props> { }
+
 type Props = $FlowFixMe; class Foo extends Component<Props> { }
 
 type Props = {||}; class Foo extends Component<Props> { }

src/rules/requireReadonlyReactProps.js

@@ -33,6 +33,7 @@ const isReactComponent = (node) => {
   );
 };
 
+// type Props = {| +foo: string |}
 const isReadOnlyObjectType = (node, {useImplicitExactTypes}) => {
   if (!node || node.type !== 'ObjectTypeAnnotation') {
     return false;
@@ -55,8 +56,21 @@ const isReadOnlyObjectType = (node, {useImplicitExactTypes}) => {
         });
 };
 
+// type Props = {| +foo: string |} | {| +bar: number |}
+const isReadOnlyObjectUnionType = (node, options) => {
+  if (!node || node.type !== 'UnionTypeAnnotation') {
+    return false;
+  }
+
+  return node.types.every((type) => {
+    return isReadOnlyObjectType(type, options);
+  });
+};
+
 const isReadOnlyType = (node, options) => {
-  return node.right.id && reReadOnly.test(node.right.id.name) || isReadOnlyObjectType(node.right, options);
+  return node.right.id && reReadOnly.test(node.right.id.name) ||
+    isReadOnlyObjectType(node.right, options) ||
+    isReadOnlyObjectUnionType(node.right, options);
 };
 
 const create = (context) => {

tests/rules/assertions/requireReadonlyReactProps.js

@@ -66,6 +66,27 @@ export default {
         },
       ],
     },
+    {
+      code: 'type Props = {| foo: string |} | {| bar: number |}; class Foo extends Component<Props> { }',
+      errors: [
+        {
+          message: 'Props must be $ReadOnly',
+        },
+      ],
+    },
+    {
+      code: 'type Props = { foo: string } | { bar: number }; class Foo extends Component<Props> { }',
+      errors: [
+        {
+          message: 'Props must be $ReadOnly',
+        },
+      ],
+      options: [
+        {
+          useImplicitExactTypes: true,
+        },
+      ],
+    },
     {
       code: 'type Props = {| +foo: string, ...bar |}; class Foo extends Component<Props> { }',
       errors: [
@@ -164,6 +185,17 @@ export default {
     {
       code: 'type Props = {| +foo: string, +bar: number |}; class Foo extends Component<Props> { }',
     },
+    {
+      code: 'type Props = {| +foo: string |} | {| +bar: number |}; class Foo extends Component<Props> { }',
+    },
+    {
+      code: 'type Props = { +foo: string } | { +bar: number }; class Foo extends Component<Props> { }',
+      options: [
+        {
+          useImplicitExactTypes: true,
+        },
+      ],
+    },
     {
       code: 'type Props = $FlowFixMe; class Foo extends Component<Props> { }',
     },