Annex D (normative) Compatibility features [depr]

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

D.4.2 Header <cstdalign> synopsis [depr.cstdalign.syn]

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