Can a C implementation implicitly include standard headers when including a different header?

Question

While reading Is this proper C declaration? If so, why does it not work? I was thinking about

#include <stdio.h>

int main(void) {
  int bool = 0;
  return bool == 0;
}

Is this program strictly conforming? In other words, is stdio.h allowed to include stdbool.h or is it forbidden to do so? Is this specified by the spec?

Answer 1

C standard headers can not include other headers. This is different from C++, where it is explicitly allowed.

C99 standard, section 7.1.3

Each header declares or defines all identifiers listed in its associated subclause[...] No other identifiers are reserved.