Re: [PATCH v3 2/3] qapi: Do not generate empty enum

[Juan Quintela <quintela@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> wrote:
> On Thu, Mar 16, 2023 at 01:31:04PM +0100, Markus Armbruster wrote:
>> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
>> 
>> > Per the C++ standard, empty enum are ill-formed. Do not generate
>> > them in order to avoid:
>> >
>> >   In file included from qga/qga-qapi-emit-events.c:14:
>> >   qga/qga-qapi-emit-events.h:20:1: error: empty enum is invalid
>> >      20 | } qga_QAPIEvent;
>> >         | ^
>> >
>> > Reported-by: Markus Armbruster <armbru@redhat.com>
>> > Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>> 
>> Two failures in "make check-qapi-schema" (which is run by "make check"):
>> 
>> 1. Positive test case qapi-schema-test
>> 
>>     --- /work/armbru/qemu/bld-x86/../tests/qapi-schema/qapi-schema-test.out
>>     +++ 
>>     @@ -19,7 +19,6 @@
>>          member enum2: EnumOne optional=True
>>          member enum3: EnumOne optional=False
>>          member enum4: EnumOne optional=True
>>     -enum MyEnum
>>      object Empty1
>>      object Empty2
>>          base Empty1
>> 
>>    You forgot to update expected test output.  No big deal.
>> 
>> 2. Negative test case union-empty
>> 
>>     --- /work/armbru/qemu/bld-x86/../tests/qapi-schema/union-empty.err
>>     +++ 
>>     @@ -1,2 +1,2 @@
>>     -union-empty.json: In union 'Union':
>>     -union-empty.json:4: union has no branches
>>     +union-empty.json: In struct 'Base':
>>     +union-empty.json:3: member 'type' uses unknown type 'Empty'
>>     stderr:
>>     qapi-schema-test FAIL
>>     union-empty FAIL
>> 
>>    The error message regresses.
>> 
>>    I can see two ways to fix this:
>> 
>>    (A) You can't just drop empty enumeration types on the floor.  To not
>>        generate code for them, you need to skip them wherever we
>>        generate code for enumeration types.
>> 
>>    (B) Outlaw empty enumeration types.
>> 
>> I recommend to give (B) a try, it's likely simpler.
>
> Possible trap-door with (B), if we have any enums where *every*
> member is conditionalized on a CONFIG_XXX rule, there might be
> certain build scenarios where an enum suddenly becomes empty.

Do we have an example for this?
Because it looks really weird.  I would expect that the "container" unit
of that enumeration is #ifdef out of compilation somehow.

Later, Juan.