DDSXTY_ — Restrictions on MAP key element type not (clearly) documented/specified

Allthough the specification of the MAP_TYPE in paragraph 7.2.2.3.4 refers to restrictions for key element type this is not formalized in either the specification for the IDL representation for maps or the C++ language mapping.

It should be clearly noted (for the IDL representation in bnf) that the key element type is restricted to integer and string types only.

The current IDL representation specs for map leave all options open for defining the key element type.
Although the specs in paragraph 7.2.2.3.4 describe the use of other element types as 'behaviour undefined' this is not clear enough as the chosen C++ representation simply will not work when f.i STRUCT or UNION types are chosen as key element type.

In section 7.2.2.3.4, provide a rationale for the restriction on key types.
In the section on the Plain Language Binding for map types (7.5.1.3), clarify that the use of an unsupported key element type shall result in a compilation error. Further indicate that in C++ (section 7.5.1.3.1), the Compare function implementation shall consider the character contents of strings; it shall not compare pointer values.