[Fiber] Support AsyncIterable children in SuspenseList (#33299)

We support AsyncIterable (more so when it's a cached form like in coming
from Flight) as children.

This fixes some warnings and bugs when passed to SuspenseList.

Ideally SuspenseList with `tail="hidden"` should support unblocking
before the full result has resolved but that's an optimization on top.
We also might want to change semantics for this for
`revealOrder="backwards"` so it becomes possible to stream items in
reverse order.

DiffTrain build for [4c6967b](4c6967b)

Author: sebmarkbage

compiled/facebook-www/REVISION

@@ -1 +1 @@
-c6c2a52ad8fb1894b03a3bb618eb57e5deca5aa0
+4c6967be290fc31182c61cfdac19915fdb16aa60

compiled/facebook-www/REVISION_TRANSFORMS

@@ -1 +1 @@
-c6c2a52ad8fb1894b03a3bb618eb57e5deca5aa0
+4c6967be290fc31182c61cfdac19915fdb16aa60

compiled/facebook-www/React-dev.classic.js

@@ -1537,7 +1537,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.2.0-www-classic-c6c2a52a-20250519";
+    exports.version = "19.2.0-www-classic-4c6967be-20250520";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-dev.modern.js

@@ -1537,7 +1537,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.2.0-www-modern-c6c2a52a-20250519";
+    exports.version = "19.2.0-www-modern-4c6967be-20250520";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-prod.classic.js

@@ -635,4 +635,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-classic-c6c2a52a-20250519";
+exports.version = "19.2.0-www-classic-4c6967be-20250520";

compiled/facebook-www/React-prod.modern.js

@@ -635,4 +635,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-modern-c6c2a52a-20250519";
+exports.version = "19.2.0-www-modern-4c6967be-20250520";

compiled/facebook-www/React-profiling.classic.js

@@ -639,7 +639,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-classic-c6c2a52a-20250519";
+exports.version = "19.2.0-www-classic-4c6967be-20250520";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-profiling.modern.js

@@ -639,7 +639,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-modern-c6c2a52a-20250519";
+exports.version = "19.2.0-www-modern-4c6967be-20250520";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/ReactART-dev.classic.js

@@ -4251,6 +4251,20 @@ __DEV__ &&
         }
       };
     }
+    function validateSuspenseListNestedChild(childSlot, index) {
+      var isAnArray = isArrayImpl(childSlot);
+      childSlot = !isAnArray && "function" === typeof getIteratorFn(childSlot);
+      return isAnArray || childSlot
+        ? ((isAnArray = isAnArray ? "array" : "iterable"),
+          console.error(
+            "A nested %s was passed to row #%s in <SuspenseList />. Wrap it in an additional SuspenseList to configure its revealOrder: <SuspenseList revealOrder=...> ... <SuspenseList revealOrder=...>{%s}</SuspenseList> ... </SuspenseList>",
+            isAnArray,
+            index,
+            isAnArray
+          ),
+          !1)
+        : !0;
+    }
     function finishQueueingConcurrentUpdates() {
       for (
         var endIndex = concurrentQueuesIndex,
@@ -8323,20 +8337,6 @@ __DEV__ &&
         propagationRoot
       );
     }
-    function validateSuspenseListNestedChild(childSlot, index) {
-      var isAnArray = isArrayImpl(childSlot);
-      childSlot = !isAnArray && "function" === typeof getIteratorFn(childSlot);
-      return isAnArray || childSlot
-        ? ((isAnArray = isAnArray ? "array" : "iterable"),
-          console.error(
-            "A nested %s was passed to row #%s in <SuspenseList />. Wrap it in an additional SuspenseList to configure its revealOrder: <SuspenseList revealOrder=...> ... <SuspenseList revealOrder=...>{%s}</SuspenseList> ... </SuspenseList>",
-            isAnArray,
-            index,
-            isAnArray
-          ),
-          !1)
-        : !0;
-    }
     function initSuspenseListRenderState(
       workInProgress,
       isBackwards,
@@ -8366,6 +8366,15 @@ __DEV__ &&
         revealOrder = nextProps.revealOrder,
         tailMode = nextProps.tail;
       nextProps = nextProps.children;
+      var suspenseContext = suspenseStackCursor.current,
+        shouldForceFallback = 0 !== (suspenseContext & ForceSuspenseFallback);
+      shouldForceFallback
+        ? ((suspenseContext =
+            (suspenseContext & SubtreeSuspenseContextMask) |
+            ForceSuspenseFallback),
+          (workInProgress.flags |= 128))
+        : (suspenseContext &= SubtreeSuspenseContextMask);
+      push(suspenseStackCursor, suspenseContext, workInProgress);
       if (
         void 0 !== revealOrder &&
         "forwards" !== revealOrder &&
@@ -8428,12 +8437,29 @@ __DEV__ &&
         !1 !== nextProps
       )
         if (isArrayImpl(nextProps))
-          for (var i = 0; i < nextProps.length; i++) {
-            if (!validateSuspenseListNestedChild(nextProps[i], i)) break a;
+          for (
+            suspenseContext = 0;
+            suspenseContext < nextProps.length;
+            suspenseContext++
+          ) {
+            if (
+              !validateSuspenseListNestedChild(
+                nextProps[suspenseContext],
+                suspenseContext
+              )
+            )
+              break a;
           }
-        else if (((i = getIteratorFn(nextProps)), "function" === typeof i)) {
-          if ((i = i.call(nextProps)))
-            for (var step = i.next(), _i = 0; !step.done; step = i.next()) {
+        else if (
+          ((suspenseContext = getIteratorFn(nextProps)),
+          "function" === typeof suspenseContext)
+        ) {
+          if ((suspenseContext = suspenseContext.call(nextProps)))
+            for (
+              var step = suspenseContext.next(), _i = 0;
+              !step.done;
+              step = suspenseContext.next()
+            ) {
               if (!validateSuspenseListNestedChild(step.value, _i)) break a;
               _i++;
             }
@@ -8443,40 +8469,31 @@ __DEV__ &&
             revealOrder
           );
       reconcileChildren(current, workInProgress, nextProps, renderLanes);
-      nextProps = suspenseStackCursor.current;
-      if (0 !== (nextProps & ForceSuspenseFallback))
-        (nextProps =
-          (nextProps & SubtreeSuspenseContextMask) | ForceSuspenseFallback),
-          (workInProgress.flags |= 128);
-      else {
-        if (null !== current && 0 !== (current.flags & 128))
-          a: for (current = workInProgress.child; null !== current; ) {
-            if (13 === current.tag)
-              null !== current.memoizedState &&
-                scheduleSuspenseWorkOnFiber(
-                  current,
-                  renderLanes,
-                  workInProgress
-                );
-            else if (19 === current.tag)
+      if (
+        !shouldForceFallback &&
+        null !== current &&
+        0 !== (current.flags & 128)
+      )
+        a: for (current = workInProgress.child; null !== current; ) {
+          if (13 === current.tag)
+            null !== current.memoizedState &&
               scheduleSuspenseWorkOnFiber(current, renderLanes, workInProgress);
-            else if (null !== current.child) {
-              current.child.return = current;
-              current = current.child;
-              continue;
-            }
-            if (current === workInProgress) break a;
-            for (; null === current.sibling; ) {
-              if (null === current.return || current.return === workInProgress)
-                break a;
-              current = current.return;
-            }
-            current.sibling.return = current.return;
-            current = current.sibling;
+          else if (19 === current.tag)
+            scheduleSuspenseWorkOnFiber(current, renderLanes, workInProgress);
+          else if (null !== current.child) {
+            current.child.return = current;
+            current = current.child;
+            continue;
           }
-        nextProps &= SubtreeSuspenseContextMask;
-      }
-      push(suspenseStackCursor, nextProps, workInProgress);
+          if (current === workInProgress) break a;
+          for (; null === current.sibling; ) {
+            if (null === current.return || current.return === workInProgress)
+              break a;
+            current = current.return;
+          }
+          current.sibling.return = current.return;
+          current = current.sibling;
+        }
       switch (revealOrder) {
         case "forwards":
           renderLanes = workInProgress.child;
@@ -19063,10 +19080,10 @@ __DEV__ &&
     (function () {
       var internals = {
         bundleType: 1,
-        version: "19.2.0-www-classic-c6c2a52a-20250519",
+        version: "19.2.0-www-classic-4c6967be-20250520",
         rendererPackageName: "react-art",
         currentDispatcherRef: ReactSharedInternals,
-        reconcilerVersion: "19.2.0-www-classic-c6c2a52a-20250519"
+        reconcilerVersion: "19.2.0-www-classic-4c6967be-20250520"
       };
       internals.overrideHookState = overrideHookState;
       internals.overrideHookStateDeletePath = overrideHookStateDeletePath;
@@ -19100,7 +19117,7 @@ __DEV__ &&
     exports.Shape = Shape;
     exports.Surface = Surface;
     exports.Text = Text;
-    exports.version = "19.2.0-www-classic-c6c2a52a-20250519";
+    exports.version = "19.2.0-www-classic-4c6967be-20250520";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&