Function reconcileChildFibersImpl

packages/react-reconciler/src/ReactChildFiber.js:1767–1944

(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChild: any,
    lanes: Lanes,
  )

  // itself. They will be added to the side-effect list as we pass through the
  // children and the parent.
  function reconcileChildFibersImpl(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChild: any,
    lanes: Lanes,
  ): Fiber | null {
    // This function is only recursive for Usables/Lazy and not nested arrays.
    // That's so that using a Lazy wrapper is unobservable to the Fragment
    // convention.
    // If the top level item is an array, we treat it as a set of children,
    // not as a fragment. Nested arrays on the other hand will be treated as
    // fragment nodes. Recursion happens at the normal flow.

    // Handle top level unkeyed fragments without refs (enableFragmentRefs)
    // as if they were arrays. This leads to an ambiguity between <>{[...]}</> and <>...</>.
    // We treat the ambiguous cases above the same.
    // We don't use recursion here because a fragment inside a fragment
    // is no longer considered "top level" for these purposes.
    const isUnkeyedUnrefedTopLevelFragment =
      typeof newChild === 'object' &&
      newChild !== null &&
      newChild.type === REACT_FRAGMENT_TYPE &&
      newChild.key === null &&
      (enableFragmentRefs ? newChild.props.ref === undefined : true);

    if (isUnkeyedUnrefedTopLevelFragment) {
      validateFragmentProps(newChild, null, returnFiber);
      newChild = newChild.props.children;
    }

    // Handle object types
    if (typeof newChild === 'object' && newChild !== null) {
      switch (newChild.$$typeof) {
        case REACT_ELEMENT_TYPE: {
          const prevDebugInfo = pushDebugInfo(newChild._debugInfo);
          const firstChild = placeSingleChild(
            reconcileSingleElement(
              returnFiber,
              currentFirstChild,
              newChild,
              lanes,
            ),
          );
          currentDebugInfo = prevDebugInfo;
          return firstChild;
        }
        case REACT_PORTAL_TYPE:
          return placeSingleChild(
            reconcileSinglePortal(
              returnFiber,
              currentFirstChild,
              newChild,
              lanes,
            ),
          );
        case REACT_LAZY_TYPE: {
          const prevDebugInfo = pushDebugInfo(newChild._debugInfo);
          const result = resolveLazy((newChild: any));