[nathanrf]

Unfortunately, C++ uses ++ and -- for iterators, many of which cannot reasonably implement += or -=. This distinction is baked into the type system to tell whether or not an iterator supports efficient "multiple advance" (e.g. a linked list iterator doesn't have += but a pointer into a contiguous vector does).

There's no way to fix this in a reverse-compatible way for existing code (which is one of the constraints of cpp2- it must work with all existing C++ so that it is possible for existing projects to migrate regardless of size).

[Rexxar]

Making ++ or -- a statement that increment the target without returning a value should probably be enough for forward iterators.

[layer8]

A noncopying ++ could be spelled `+= 1` though. So some iterators would support `+= 1`, but not `+= 2`. This would be vaguely similar to how the null pointer constant was defined as an integer constant expression that evaluates to zero: Define `+= <integer constant expression that evaluates to one>` as the increment operator.