DDSXTY12 — Allow empty structures

Allow empty structures as part of the XTypes spec. XTYPES spec it does not require structures to have any members. It is not explicit about this but it does say they
can have any number of members (including zero). The IDL syntax. XTYPES uses IDL as one of the ways to express types so if the IDL grammar specifies made empty structures
illegal we would have an inconsistency between what the X-TYPES type-system says and what the IDL grammar supports...
Looking at the IDL syntax in the X-Types it defines:
<struct_type> ::= <struct_header> “

”

The <member_list> refers to the production on the IDL spec (X-TYPES
only lists the new productions or the ones that are modified from
IDL), the <struct_type> is modified in XTYPES to support inheritance,
hence its appearance in the XTYPES duc but the <member_list> is
unchanged.

In IDL 3.5 <member_list> is defined as:

<member_list>::=<member>+

Which does seem to indicate that empty structures are not supported.

The change should be applied to IDL 3.5/IDL4

Supporting empty structures is desired to facilitate the mapping from other data-models that may support them in that sense the type system in XTYPES is already doing the right thing and it is IDL the one that needs to change...

Given that the plan is to remove the IDL syntax extensions from XTYPES and have XTTPES reference IDL4+ instead. It is best to close this issue here with "NoChange" and instead file an issue with IDL4 to add support for empty structures.