The "big bug" indeed limits the possibilities of _Generic. But if you really want to abuse it beyond its intended scope, I think you can simply add some casting

The "big bug" indeed limits the possibilities of _Generic. But if you really want to abuse it beyond its intended scope, I think you can simply add some casting, and not the gimmicks with nested _Generic. #define length(x) _Generic(x, \ char * : strlen((char*)(x)), \ String * : ((String*)(x))->len \ )