[Someone]

C++ doesn't have the friendliest grammar for addressing potential parsing ambiguities.

As an example, your proposed solutions

   _1,000,000
   k1,000,000

Both are already valid if variables or functions _1 or k1 are in scope, and evaluate to (octal) zero.

And I haven't checked, but it would guess that invalidating "x=1,2" in the parser would not be "simple". I cannot think of a reason, but it also might turn out to be more common then one would think due to macro expansions.