Witnessing -Wc99-designator with xt-clang (clang-10) with enabling -Wall, -Werror

Introduction

Extension to the discussion filed here: https://github.com/zephyrproject-rtos/zephyr/issues/33549

Problem description

The following MACROs to init a struct are mixing designated and non-designated initializers. Note, the initialization of {.obj = _event_obj}

This triggers -Wc99-designator error.

#define K_POLL_EVENT_INITIALIZER(_event_type, _event_mode, _event_obj) \
    { \
    .poller = NULL, \
    .type = _event_type, \
    .state = K_POLL_STATE_NOT_READY, \
    .mode = _event_mode, \
    .unused = 0, \
    { \
        .obj = _event_obj, \
    }, \
    }

#define K_POLL_EVENT_STATIC_INITIALIZER(_event_type, _event_mode, _event_obj, \
                    event_tag) \
    { \
    .tag = event_tag, \
    .type = _event_type, \
    .state = K_POLL_STATE_NOT_READY, \
    .mode = _event_mode, \
    .unused = 0, \
    { \
        .obj = _event_obj, \
    }, \
    }

Proposed change

The error can be fixed up by explicitly introducing a concrete variable for anonymous union and initialize with designated initializer

Detailed RFC

With this the union in new struct k_poll_event can now be

struct k_poll_event {
// ignoring other variables ...... 

    /** per-type data */
    union {
        void *obj;
        struct k_poll_signal *signal;
        struct k_sem *sem;
        struct k_fifo *fifo;
        struct k_queue *queue;
        struct k_msgq *msgq;
#ifdef CONFIG_PIPES
        struct k_pipe *pipe;
#endif
    } k_obj;
};

So the macros will now be:

#define K_POLL_EVENT_INITIALIZER(_event_type, _event_mode, _event_obj) \
    { \
    .poller = NULL, \
    .type = _event_type, \
    .state = K_POLL_STATE_NOT_READY, \
    .mode = _event_mode, \
    .unused = 0, \
    .k_obj = { \
        .obj = _event_obj, \
    }, \
    }

#define K_POLL_EVENT_STATIC_INITIALIZER(_event_type, _event_mode, _event_obj, \
                    event_tag) \
    { \
    .tag = event_tag, \
    .type = _event_type, \
    .state = K_POLL_STATE_NOT_READY, \
    .mode = _event_mode, \
    .unused = 0, \
    .k_obj = { \
        .obj = _event_obj, \
    }, \
    }

Dependencies

None

Concerns and Unresolved Questions

needs changing across multiple files
Going via alternatives below, can affect some compilers.

Alternatives

Directly initialize anonymous .obj variable. c99 allows this way since, the nest .obj is unique in global struct k_poll_event struct.

With GCC 11.3.0 and -std=c++20 this error occurs:

.../include/zephyr/kernel.h:5707:9: error: either all initializer clauses should be designated or none of them should be
 5707 |         { \
      |         ^
...file.cpp:325:9: note: in expansion of macro ‘K_POLL_EVENT_STATIC_INITIALIZER’
  325 |         K_POLL_EVENT_STATIC_INITIALIZER(K_POLL_TYPE_MSGQ_DATA_AVAILABLE, K_POLL_MODE_NOTIFY_ONLY,

Code example:

void does_not_compile()
{
    struct k_poll_event events[] = {
        K_POLL_EVENT_STATIC_INITIALIZER(K_POLL_TYPE_MSGQ_DATA_AVAILABLE, K_POLL_MODE_NOTIFY_ONLY,
                                        nullptr, 0),
    };
}

And the error is valid, since C++ does not allow mixing the initializes.

For C++, using __cplusplus #define to strip the extra brackets out could do it. But that would not be a fix for clang -Wc99-designator error. Is there some xt-xcc define which could be utilized instead?

zephyrproject-rtos / zephyr