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

["Daniel P. Berrangé" <berrange@redhat.com>]

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.

With regards,
Daniel
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