Annex D (normative) Compatibility features [depr]

D.6 C standard library headers [depr.c.headers]

D.6.4 Header <stdbool.h> synopsis [depr.stdbool.h.syn]

#define __bool_true_false_are_defined 1
The contents of the C++ header <stdbool.h> are the same as the C standard library header <stdbool.h>, with the following changes: The header <stdbool.h> does not define macros named bool, true, or false.
See also: ISO C 7.18