Warn about setState in render

scripts/fiber/tests-passing-except-dev.txt

@@ -106,7 +106,6 @@ src/renderers/__tests__/ReactComponentTreeHook-test.native.js
 * does not report top-level wrapper as a root
 
 src/renderers/__tests__/ReactCompositeComponent-test.js
-* should warn about `setState` in render
 * should warn about `setState` in getChildContext
 * should disallow nested render calls
 

scripts/fiber/tests-passing.txt

@@ -577,6 +577,7 @@ src/renderers/__tests__/ReactCompositeComponent-test.js
 * should warn about `forceUpdate` on unmounted components
 * should warn about `setState` on unmounted components
 * should silently allow `setState`, not call cb on unmounting components
+* should warn about `setState` in render
 * should cleanup even if render() fatals
 * should call componentWillUnmount before unmounting
 * should warn when shouldComponentUpdate() returns undefined

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -102,6 +102,16 @@ if (__DEV__) {
       ctor && (ctor.displayName || ctor.name) || 'ReactClass'
     );
   };
+
+  var warnAboutUpdateInRender = function(instance : ReactClass<any>) {
+    warning(
+      ReactCurrentOwner.current == null,
+      'Cannot update during an existing state transition (such as within ' +
+      '`render` or another component\'s constructor). Render methods should ' +
+      'be a pure function of props and state; constructor side-effects are ' +
+      'an anti-pattern, but can be moved to `componentWillMount`.'
+    );
+  };
 }
 
 var timeHeuristicForUnitOfWork = 1;
@@ -1102,6 +1112,13 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       nextUnitOfWork = null;
     }
 
+    if (__DEV__) {
+      if (fiber.tag === ClassComponent) {
+        const instance = fiber.stateNode;
+        warnAboutUpdateInRender(instance);
+      }
+    }
+
     let node = fiber;
     let shouldContinue = true;
     while (node !== null && shouldContinue) {