Shadowing at AST level with the lexical scope is easy to implement, it's just each usage looks up inside out to parent scopes. But if we treat each assignment as a kind of shadowing, it works in a similar way and turns into a kind of SSA. The complexity arises with phi-nodes when multiple paths join. I think the confusion comes from the strict definition of SSA as something useful for the very late stage in the pipeline, but the same concept can exist much earlier in the pipeline.