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A general description is here.
In R 4.0.0, ‘sort.list(x)’ when ‘is.object(x)’ was true, e.g., for ‘x <- I(letters)’, was accidentally using ‘method = "radix"’. Consequently, e.g., ‘merge(<data.frame>)’ was much slower than previously; reported in PR#17794.
‘sort.list(x)’ now works for non-atomic objects ‘x’ and ‘method = "auto"’ (the default) or ‘"radix"’ in cases ‘order(x)’ works, typically via a ‘xtfrm()’ method.