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

[Markus Armbruster <armbru@redhat.com>]

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.