A union or alternate without branches makes no sense and doesn't work:
it can't be instantiated. A union or alternate with just one branch
works, but is degenerate. We accept the former, but reject the
latter. Weird. docs/devel/qapi-code-gen.txt doesn't mention the
difference. It claims an alternate definition is "is similar to a
simple union type".
Permit degenerate alternates to make them consistent with unions.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-10-armbru@redhat.com>
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