[Bug c/126003] integer overflow during compile-time arithmetic is not always detected

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126003

--- Comment #3 from Drea Pinski <pinskia at gcc dot gnu.org> ---
> According to the syntax of the C23 standard, neither is parsed as a constant expression.

Huh? 
`TWOGIG + TWOGIG`  is one because TWOGIG  is a macro that gets subsituted to
`2000000000`

That is why:
int x = TWOGIG+TWOGIG;

is allowed be compiled in the first place, since global variables require
constant expressions for their initilizer.