Annex D (normative) Compatibility features [depr]

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

D.6.3 Header <stdalign.h> synopsis [depr.stdalign.h.syn]

#define __alignas_is_defined 1
The contents of the C++ header <stdalign.h> are the same as the C standard library header <stdalign.h>, with the following changes: The header <stdalign.h> does not define a macro named alignas.
See also: ISO C 7.15