I prefer the rule in my own languages of "no arithmetic expressions whose meaning can be changed by adding parentheses". So `x + y - z` is allowed but `x - y + z` is not.
If operators are over-loadable, you support floats (in a non ffast-math mode), or you treat overflow in most non-modular-arithmetic ways, (x + y) - z and x + (y - z) are different.
Maybe it's worth saying "they're close enough to the same that parentheses should be optional", but I can definitely see the argument for just requiring them regardless.
Valid point, though my current language supports neither overloading, floats, nor non-modular integer overflow, and parentheses are permissible where not required (for extra-semantical cases where order does matter) :)
