Underscores are already being used for user defined literals.

User-defined literals can't generally start with a digit, so it should be perfectly fine to allow 1_000_000 syntax for 1000000. Or I'm forgetting something?

Hexadecimal numbers, probably. 0x001_f05 would be ambiguous.

I don't think that's it. 0x001_f05 starts with 0, which is still not something an identifier can do.

0x001_f05 could be a UDL named f05 which should be passed 0x001.

Oh. UDLs are a new one to me. I didn't know that was in the language.

I stand corrected.

Where do you get this information from, because it's entirely incorrect.

The "already an operator" can be a factor in the design reasoning but if they really really wanted to, they could have made the comma syntax for digit separators by modifying the parsing grammar: e.g. a special prefix and/or contextual parsing.

For example, the "->" was already an operator for pointer indirection but it was reused for lambda definitions. The "[]" was already used for array subscript indexes but was reused for lambda captures.

You can't do that for ",", because it will break the existing code. For example, int x = 0; x = 1,2; is perfectly valid pre c++11 code. This will assign 1 to x. Now if you make "," as digit separator, it will assign 12 to x

I mentioned the possibility of a special prefix in my previous post. In other words, that parsing ambiguity goes away if the standards committee wanted to define a special prefix such as 'd', 'k', or '_' to inform the parser that the next comma is a "digit separator" instead of "comma operator" such as _1,000,000 . E.g. there's already intelligence about commas in the parser to disambiguate function calls with commas as in "repositionxyz(914,348,122)"

Or, they could have defined C++14 to simply invalidate your example syntax of "x=1,2". As an example, the "auto" keyword was made a "breaking change" such that "for (auto int i = 0;;)" no longer compiles.

The bottom line is that there are a myriad of ways to address (potential) parsing ambiguities when introducing new language features and syntax. (Whether or not a comma digit separator is worth the clumsier syntax of a special prefix or inflicting the pain of a breaking change is a separate concept.)

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.