It also makes it clear to users whether they've just run up against a
limitation of the implementation they're using or whether what they've
written is genuinely illegal code. They are NOT the same thing.
Attempting to conflate them into one exception for the mere sake of
not adding a new exception type is a false economy. Take it to
python-dev if you want, but I will strongly oppose this check going in
with it raising an ordinary SyntaxError instead of a new subclass.