https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
--- Comment #7 from Martin Uecker <muecker at gwdg dot de> --- An attribute is certainly simpler and should be easy to add. I proposed similar extension for C23 and there was some interest, but I did not have time to follow up. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2660.pdf Sizeof is not a constant expression in ISO C for a VLA and it is not a constant expression if the struct contains a VLA (GNU extension). So this is already the case and nothing would need to change. It would also potentially avoid mistakes when computing the size of such a struct. But the rules for initialization are not so clear. I do not think it is a good idea to differentiate between file scope structs and others. This would be confusing. Considering that the GNU extensions is rarely used, one could consider redefining the meaning of int n = 1; struct { int n; char buf[n]; }; so that the 'n' refers to the member. Or we add a new syntax similar to designators (which intuitively makes sense to me).
