Annex D (normative) Compatibility features [depr]

D.4 C++ standard library headers [depr.cpp.headers]

D.4.3 Header <cstdbool> synopsis [depr.cstdbool.syn]

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