What's the expected behavior of open(name, O_CREAT|O_DIRECTORY, mode)?

Question

Despite a careful read of the related standard documentation, I can't understand what's the expected behavior in POSIX compliant systems when a open system call is invoked with flags including O_CREAT|O_DIRECTORY.

The standard specifies that

If O_CREAT and O_DIRECTORY are set and the requested access mode is neither O_WRONLY nor O_RDWR, the result is unspecified.

However it does not specify the behavior of the system with neither (O_CREAT|O_DIRECTORY|O_WRONLY) nor (O_CREAT|O_DIRECTORY|O_RDWR). Indeed (as far as I can understand) the behavior on EISDIR only apply to existing directories.

In the section related to O_CREATE, the standard specifies that, when the named file does not exist,

if O_DIRECTORY is not set the file shall be created as a regular file; [...]

but again it does not specify what will happen if O_DIRECTORY is set too.

I've looked the manual pages of both NetBSD (which notoriously cares a lot about POSIX compliance) and Linux (which is a widely used system, despite not actually a POSIX one) but I can't find any clarification.

Is it correct to say that the use of both flags is unspecified? And if so, what's the most common behavior?

Is open(name, O_CREAT|O_DIRECTORY, mode) equivalent to mkdir on any POSIX compliant OS?