Currently [stdalign.h.syn] states

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/IEC 9899:2018, 7.15

which implicitly specifies that __alignof_is_defined is also provided in C++, because C17 specified that the macro is provided in <stdaligh.h>.

However, there's no explicit mentioning of __alignof_is_defined in the C++ standard wording. And [depr.c.macros]/1 (added by LWG 3827) seemingly contradicts with [stdalign.h.syn] and only makes __alignas_is_defined deprecated.

It seems that we should explicitly mention __alignof_is_defined in [depr.c.macros] at this moment.