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Philippe Verdy wrote:

Note that that page of documentation uses an usual non-sense notation to describe the content of the local parameter named "arg" (implicitly declared in all functions):
arg={5, 8; n=2}
This notation uses a semicolon in a table constructor, a syntax that does not exist in Lua.
It does exist in lua. In tables one can use either ',' or ';' as separator as stated in the manual:,
It may suggest that this semicolon indicates that the key/value pair n=2 is not stored in the table itself, but in its metatable.
It was used in earlier versions of lua to separate array and key-value parts. (see
Such observation cannot be made, but if it does, then any declared function that explicitly accesses to its "arg" parameter could have to create (on function entry) a new table indexing all upvalues (including nil values), and another table to store the key/value pair with an implicit key name 'n' (here ['n'] = 2) containing the effective number of upvalues (including nil's).

Such creation of two tables would be very inefficient (stressing the garbage collector), so I suspect that instead the "arg" type is not a true "table" but behaves "like a table" in user's Lua code.
Look at the sources. Here a normal table is created for the args table without a metatable. There is no special type for the args table nor has it only an array part.

I do not comment on the further points since they build upon these wrong asumptions.