Annex G (informative) Ill-formed, no diagnostic required [ifndr]

G.2 [lex]: Lexical conventions [ifndr.lex]

G.2.1[ifndr:lex.name.reserved]
Specified in: [lex.name]

1
#
Using an identifier reserved for use by C++ is ill-formed, no diagnostic required.
2
#
[Example 1: int _z; // IFNDR, z is reserved because it starts with at global scope int main() { int __x; // IFNDR, x is reserved because it starts with int _Y; // IFNDR, Y is reserved because it starts with followed by a capital letter int x__y; // IFNDR, xy is reserved because it contains } — end example]