The code labelled “Block 1” will run when the tree is empty, setting the current stack frame’s result value to the frame’s accumulator argument, and returning control back to this frame’s parent (by changing the active frame to point at the current frame’s parent). This is identical to the behavior of the recursive implementation.
So, how can we solve this? One way is to explicitly pass the inner serializer provider as a type parameter directly to SerializeIterator. We will call this pattern higher-order providers, because SerializeIterator now has a generic parameter specifically for the item serializer. With this in place, our SerializeIterator implementation can now require that SerializeItem also implements SerializeImpl, using the iterator's Item as the value type.，详情可参考wps
。关于这个话题，谷歌提供了深入分析
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