Intentional. See
https://github.com/apache/calcite/blob/98f3048fb1407e2878162ffc80388d4f9dd094b2/core/src/main/java/org/apache/calcite/sql/ddl/package-info.java#L25:
* If you are writing a project that requires DDL it is likely that your
* DDL syntax is different than ours. We recommend that
I notice that SqlColumnDeclaration has a non-public constructor, which makes
it impossible to extend from a different package.
Is this design intentional or is it a bug?
(There are some good reasons for extending this class. For example, a
comment on the class says "FOREIGN KEY when we
