Fix compiler warning when compling NSFontAssetRequest.m in libs-gui

[qmfrederik]

Cast the result type of CALL_NON_NULL_BLOCK to void to satisfy the ? operator.

If not, compiling on clang on msys2/ucrt64 fails with this error:

NSFontAssetRequest.m:48:3: error: incompatible operand types ('int' and 'id')
    48 |   CALL_BLOCK(completionHandler, error);
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/tools/msys64/ucrt64/include/GNUstepBase/GSBlocks.h:110:53: note: expanded from macro 'CALL_BLOCK'
    110 | #define CALL_BLOCK(block, args...) ((NULL != block) ? CALL_NON_NULL_BLOCK(block, args) : nil)
        |                                                     ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   ~~~
1 error generated.

[rfm]

I don't think that can be correct, because the result of the call can be assigned (which is impossible for void).
Of course this is designed to be used where we don't have real blocks, and to support a subset of the possible return types.
So presumably we want to cast to 'id' rather than 'void' (or perhaps cast both sides to 'void*')?

[rfm]

Thanks for the update.

[rfm]

Oops, failed to notice that the compiler objects. I wonder why?

[rfm]

I guess the problem is that the block may return a value or may not return a value at all.
If so, the idea of having a simple macro to cope with all cases may be fundamentally broken.
Possibly we need one macro to call a block which doesn't return a value, and a separate macro to call a block which does return a value.
Something like:

#define CALL_BLOCK(block, args...) ({if (NULL != block) CALL_NON_NULL_BLOCK(block, args);})
#define CALL_BLOCK_RET(block, rettype, args...) ((NULL != block) ? (rettype)CALL_NON_NULL_BLOCK(block, args) : (rettype)0)

Does that seem reasonable?
Of course existing code using the macro may need to be changed.

[rfm]

In th absence of feedback, I went ahead with that last proposal.