Undecoded returns all keys that have not been decoded in the order in which they appear in the original TOML document.
This includes keys that haven't been decoded because of a Primitive value. Once the Primitive value is decoded, the keys will be considered decoded.
Also note that decoding into an empty interface will result in no decoding, and so no keys will be considered decoded.
In this sense, the Undecoded keys correspond to keys in the TOML document that do not have a concrete type in your representation.