Comparing enum / define in keil ARM compiler

Question

I'm facing difficulties to make the following conditional compilation working:

enum {  MIDI_USB_DEV_MCU_IDX, MIDI_WC_BLE_IDX, MIDI_CONN_1_IDX, MIDI_CONN_2_IDX,MIDI_IN_OUT_UARTS_NB 
};

#define MIDI_UART_IDX_TO_USE_SYSEXS      MIDI_WC_BLE_IDX

I use the following code to remind me the above define should not be used in Release mode, but the compiler does not warn on the below test.

#if MIDI_UART_IDX_TO_USE_SYSEXS != MIDI_USB_DEV_MCU_IDX
#warning MIDI_UART_IDX_TO_USE_SYSEXS must be MIDI_USB_DEV_MCU_IDX in RELEASE MODE
#endif

From what I read the enums are treated as Int, using the enum in my source works fine but the conditional compiler test fails. Any help or tip would be appreciated Thanks.

Answer 1

Enumeration constants do not exist in precompilation phase. Your condition expression will expand to

#if 0 != 0

Only way is to make constant work in #if expressions is to define macro constant.

#define MIDI_USB_DEV_MCU_IDX (0)
#define MIDI_WC_BLE_IDX      (1)
#define MIDI_CONN_1_IDX      (2)
#define MIDI_CONN_2_IDX      (3)
#define MIDI_IN_OUT_UARTS_NB (4)

If you are okay with compilation failing on error, then you can use enumeration constants with static assertion:

_Static_assert(
    IDI_UART_IDX_TO_USE_SYSEXS == MIDI_USB_DEV_MCU_IDX, 
    "MIDI_UART_IDX_TO_USE_SYSEXS must be MIDI_USB_DEV_MCU_IDX in RELEASE MODE");