Use multiple checks instead of forking the entire function

Easier to follow this way, and less chance the two paths will get
out of sync.

Author: acdlite

src/renderers/shared/fiber/ReactChildFiber.js

@@ -64,7 +64,6 @@ const {
   FunctionalComponent,
   ClassComponent,
   HostText,
-  HostRoot,
   HostPortal,
   CoroutineComponent,
   YieldComponent,
@@ -1103,10 +1102,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     // not as a fragment. Nested arrays on the other hand will be treated as
     // fragment nodes. Recursion happens at the normal flow.
 
-    if (ReactFeatureFlags.disableNewFiberFeatures) {
+    const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
+
+    // Handle object types
+    if (typeof newChild === 'object' && newChild !== null) {
       // Support only the subset of return types that Stack supports. Treat
       // everything else as empty, but log a warning.
-      if (typeof newChild === 'object' && newChild !== null) {
+      if (disableNewFiberFeatures) {
         switch (newChild.$$typeof) {
           case REACT_ELEMENT_TYPE:
             return placeSingleChild(reconcileSingleElement(
@@ -1116,6 +1118,40 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
               priority
             ));
 
+          case REACT_PORTAL_TYPE:
+            return placeSingleChild(reconcileSinglePortal(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+        }
+      } else {
+        switch (newChild.$$typeof) {
+          case REACT_ELEMENT_TYPE:
+            return placeSingleChild(reconcileSingleElement(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+
+          case REACT_COROUTINE_TYPE:
+            return placeSingleChild(reconcileSingleCoroutine(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+
+          case REACT_YIELD_TYPE:
+            return placeSingleChild(reconcileSingleYield(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+
           case REACT_PORTAL_TYPE:
             return placeSingleChild(reconcileSinglePortal(
               returnFiber,
@@ -1125,7 +1161,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
             ));
         }
       }
+    }
 
+    if (disableNewFiberFeatures) {
+      // The new child is not an element. If it's not null or false,
+      // and the return fiber is a composite component, throw an error.
       switch (returnFiber.tag) {
         case ClassComponent: {
           if (__DEV__) {
@@ -1152,35 +1192,6 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           );
         }
       }
-
-      if (typeof newChild === 'string' || typeof newChild === 'number') {
-        return placeSingleChild(reconcileSingleTextNode(
-          returnFiber,
-          currentFirstChild,
-          '' + newChild,
-          priority
-        ));
-      }
-
-      if (isArray(newChild)) {
-        return reconcileChildrenArray(
-          returnFiber,
-          currentFirstChild,
-          newChild,
-          priority
-        );
-      }
-
-      if (getIteratorFn(newChild)) {
-        return reconcileChildrenIterator(
-          returnFiber,
-          currentFirstChild,
-          newChild,
-          priority
-        );
-      }
-
-      return deleteRemainingChildren(returnFiber, currentFirstChild);
     }
 
     if (typeof newChild === 'string' || typeof newChild === 'number') {
@@ -1192,69 +1203,33 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       ));
     }
 
-    if (typeof newChild === 'object' && newChild !== null) {
-      switch (newChild.$$typeof) {
-        case REACT_ELEMENT_TYPE:
-          return placeSingleChild(reconcileSingleElement(
-            returnFiber,
-            currentFirstChild,
-            newChild,
-            priority
-          ));
-
-        case REACT_COROUTINE_TYPE:
-          return placeSingleChild(reconcileSingleCoroutine(
-            returnFiber,
-            currentFirstChild,
-            newChild,
-            priority
-          ));
-
-        case REACT_YIELD_TYPE:
-          return placeSingleChild(reconcileSingleYield(
-            returnFiber,
-            currentFirstChild,
-            newChild,
-            priority
-          ));
-
-        case REACT_PORTAL_TYPE:
-          return placeSingleChild(reconcileSinglePortal(
-            returnFiber,
-            currentFirstChild,
-            newChild,
-            priority
-          ));
-      }
-
-      if (isArray(newChild)) {
-        return reconcileChildrenArray(
-          returnFiber,
-          currentFirstChild,
-          newChild,
-          priority
-        );
-      }
+    if (isArray(newChild)) {
+      return reconcileChildrenArray(
+        returnFiber,
+        currentFirstChild,
+        newChild,
+        priority
+      );
+    }
 
-      if (getIteratorFn(newChild)) {
-        return reconcileChildrenIterator(
-          returnFiber,
-          currentFirstChild,
-          newChild,
-          priority
-        );
-      }
+    if (getIteratorFn(newChild)) {
+      return reconcileChildrenIterator(
+        returnFiber,
+        currentFirstChild,
+        newChild,
+        priority
+      );
     }
 
-    if (typeof newChild === 'undefined') {
+    if (!disableNewFiberFeatures && typeof newChild === 'undefined') {
+      // If the new child is undefined, and the return fiber is a composite
+      // component, throw an error. If Fiber return types are disabled,
+      // we already threw above.
       switch (returnFiber.tag) {
-        case HostRoot:
-          // TODO: Top-level render
-          break;
         case ClassComponent: {
           if (__DEV__) {
             const instance = returnFiber.stateNode;
-            if (instance.render._isMockFunction && typeof newChild === 'undefined') {
+            if (instance.render._isMockFunction) {
               // We allow auto-mocks to proceed as if they're returning null.
               break;
             }