[New] Support "detect" option for React version setting

lib/util/version.js

@@ -4,6 +4,7 @@
  */
 'use strict';
 
+const resolve = require('resolve');
 const log = require('./log');
 
 let warnedForMissingVersion = false;
@@ -12,7 +13,23 @@ function getReactVersionFromContext(context) {
   let confVer = '999.999.999';
   // .eslintrc shared settings (http://eslint.org/docs/user-guide/configuring#adding-shared-settings)
   if (context.settings.react && context.settings.react.version) {
-    confVer = context.settings.react.version;
+    let settingsVer = context.settings.react.version;
+    if (settingsVer === 'detect') {
+      try {
+        const reactPath = resolve.sync('react', {basedir: process.cwd()});
+        const react = require(reactPath);
+        settingsVer = react.version;
+      } catch (e) {
+        if (e.code === 'MODULE_NOT_FOUND') {
+          log('Warning: React version was set to "detect" in eslint-plugin-react settings, ' +
+            'but the "react" package is not installed. Assuming latest React version for linting.');
+          settingsVer = '999.999.999';
+        } else {
+          throw e;
+        }
+      }
+    }
+    confVer = settingsVer;
   } else if (!warnedForMissingVersion) {
     log('Warning: React version not specified in eslint-plugin-react settings. ' +
       'See https://github.com/yannickcr/eslint-plugin-react#configuration.');

package.json

@@ -29,7 +29,8 @@
     "has": "^1.0.3",
     "jsx-ast-utils": "^2.0.1",
     "object.fromentries": "^2.0.0",
-    "prop-types": "^15.6.2"
+    "prop-types": "^15.6.2",
+    "resolve": "^1.8.1"
   },
   "devDependencies": {
     "babel-eslint": "^8.2.5",