enum foo foo_test(void);

enum foo {
    FOO_0
};

enum bar {
    BAR_0
};

int main(int argc, char **argv)
{
    printf("sizeof(enum foo) %zu, sizeof(enum bar) %zun",
           sizeof(enum foo), sizeof(enum bar));
    return 0;
}

% make 2> errors.log
% grep -A 3 "ISO C forbids forward references to 'enum' types" errors.log