There is __builtin_constant_p (a GCC extension): https://gcc.gnu.org/onlinedocs/gcc-11.3.0/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fconstant_005fp ht

There is __builtin_constant_p (a GCC extension): https://gcc.gnu.org/onlinedocs/gcc-11.3.0/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fconstant_005fp https://gcc.gnu.org/onlinedocs/gcc-11.3.0/gcc/Other-Builtins... It's not exactly the same thing, but it's probably what you were thinking of.