Flow: use implicit exact object types

Flow standard is now to use `{ }` for strict objects and `{ key: value, ... }` for open objects, see: facebook/relay@6fa0b0d and facebook/react-native@00cfb0f

This change should be without any problems as longs as users of our NPM libraries are using Flow with `exact_by_default=true` (which is now pretty standard I'd say).

Here is how object types behave before this change:

```
type A = { x: number }; // already "exact" type but disallowed because it would be confusing to mix different syntaxes
type B = { x: number, ... }; // this is inexact
type C = {| x: number |}; // this is explicitly exact and the only allowed way how to describe type "exactness"
```

Here is how object types behave _after_ this change:

```
type A = { x: number }; // this is the only allowed syntax for "exact" type
type B = { x: number, ... }; // this is still inexact
type C = {| x: number |}; // this is also exact but no longer allowed (so it's not confusing)
```

Some related (non-blocking) issues:

- gajus/eslint-plugin-flowtype#467
- facebook/flow#8612

adeira-source-id: 5f0c905ae627f670804581f78c1b570f3f71a1e6

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Unreleased
 
+**Breaking changes ahead!**
+
+Note for all Flow users: all projects in [`adeira/universe`](https://github.com/adeira/universe) now use implicit exact Flow types (`{}` for strict objects and `{ ... }` for open objects, syntax `{||}` is deprecated). We do not expect any issues as long as you are using `exact_by_default=true` Flow option.
+
+Because of this migration of [`adeira/universe`](https://github.com/adeira/universe) to implicit exact objects (step 4 in this article: https://medium.com/flow-type/on-the-roadmap-exact-objects-by-default-16b72933c5cf) we are changing the following rules:
+
+```text
+flowtype/require-exact-type               OFF -> ERROR with "never"
+flowtype/require-inexact-type             ERROR -> OFF
+flowtype/require-readonly-react-props     ERROR -> ERROR with "useImplicitExactTypes:true"
+```
+
+In case you want to prolong the support for explicit exact objects you can simply reverse these rules and you should be good to go.
+
 # 5.3.0
 
 - Rule [`react/no-unstable-nested-components`](https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/no-unstable-nested-components.md) enabled (warnings or errors in strict mode)

__tests__/__fixtures__/valid-eslint-examples/react/jsx-no-bind.js

@@ -9,10 +9,10 @@ react/no-multi-comp
 
 import { PureComponent, Component, useState, type Element } from 'react';
 
-type LetterProps = {|
+type LetterProps = {
   +letter: string,
   +onClick: () => void,
-|};
+};
 
 const A = 65; // ASCII character code
 
@@ -23,12 +23,12 @@ class Letter extends PureComponent<LetterProps> {
   }
 }
 
-type AplhabetProps = {||};
+type AplhabetProps = {};
 
-type AplhabetState = {|
+type AplhabetState = {
   justClicked: null | string,
   letters: $ReadOnlyArray<string>,
-|};
+};
 
 export class Alphabet1 extends Component<AplhabetProps, AplhabetState> {
   constructor(props: AplhabetProps) {

__tests__/__fixtures__/valid-eslint-examples/react/no-unused-state.js

@@ -3,14 +3,14 @@
 // eslint-disable-next-line import/no-extraneous-dependencies
 import { createContext, Component, type Node, type Context, type Element } from 'react';
 
-type Props = {|
+type Props = {
   +accessToken?: string,
   +children: Node,
-|};
+};
 
-type State = {|
+type State = {
   accessToken: ?string,
-|};
+};
 
 const MyContext: Context<State> = createContext({
   accessToken: undefined,

__tests__/__fixtures__/valid-eslint-examples/react/sort-comp.js

@@ -6,8 +6,8 @@ const React = {
   Component: class Component<S, P> {},
 };
 
-type Props = {||};
-type State = {||};
+type Props = {};
+type State = {};
 
 export default class MyComponent extends React.Component<void, State> {
   props: Props;

__tests__/__snapshots__/index.test.js.snap

@@ -159,11 +159,19 @@ Object {
     "flowtype/object-type-curly-spacing": "off",
     "flowtype/object-type-delimiter": "off",
     "flowtype/require-compound-type-alias": 0,
-    "flowtype/require-exact-type": 0,
+    "flowtype/require-exact-type": Array [
+      1,
+      "never",
+    ],
     "flowtype/require-indexer-name": 0,
-    "flowtype/require-inexact-type": 2,
+    "flowtype/require-inexact-type": 0,
     "flowtype/require-parameter-type": 0,
-    "flowtype/require-readonly-react-props": 2,
+    "flowtype/require-readonly-react-props": Array [
+      2,
+      Object {
+        "useImplicitExactTypes": true,
+      },
+    ],
     "flowtype/require-return-type": 0,
     "flowtype/require-types-at-top": 0,
     "flowtype/require-valid-file-annotation": Array [
@@ -1032,6 +1040,11 @@ Snapshot Diff:
 - Value for stable rules
 + Value for STRICT rules
 
+@@ --- --- @@
+      "flowtype/require-exact-type": Array [
+-       1,
++       2,
+        "never",
 @@ --- --- @@
       "flowtype/use-flow-type": 1,
 -     "flowtype/use-read-only-spread": 1,

ourRules.js

@@ -291,11 +291,16 @@ module.exports = ({
     },
   ],
   'flowtype/require-compound-type-alias': OFF,
-  'flowtype/require-exact-type': OFF,
+  'flowtype/require-exact-type': [NEXT_VERSION_ERROR, 'never'], // we are using `exact_by_default=true`
   'flowtype/require-indexer-name': OFF,
-  'flowtype/require-inexact-type': ERROR,
+  'flowtype/require-inexact-type': OFF,
   'flowtype/require-parameter-type': OFF,
-  'flowtype/require-readonly-react-props': ERROR,
+  'flowtype/require-readonly-react-props': [
+    ERROR,
+    {
+      useImplicitExactTypes: true,
+    },
+  ],
   'flowtype/require-return-type': OFF,
   'flowtype/require-types-at-top': OFF,
   'flowtype/require-valid-file-annotation': [ERROR, 'always'],