set(x for x in range(2),) can be interpreted as set(x for x in (range(2),)).

Wouldn't be better to forbid such ambiguous syntax? The trailing comma in argument list is supported because it helps to add new arguments (or temporary comment out arguments).

    foo(x,
        y,
        #z,
       )

But set(x for x in range(2),) is not syntactically valid if add an argument after the comma. Parenthesis around a generator expression can be omitted only if it is the only argument in a function call. I think that it would be better to forbid a trailing comma in this case.