Replace the dummy arg from compound initializers in the C API with more macro magic

The dummy argument {fmt_int} was there as a workaround for MSVC and to
avoid needing to rely on another GCC extension for zero-size arrays. Use
macro magic similar to FMT_NARG() instead that replaces the problematic
zero-size array with NULL.

Signed-off-by: Ferdinand Bachmann <ferdinand.bachmann@yrlf.at>

Author: Ferdi265

include/fmt/fmt-c.h

@@ -134,6 +134,12 @@ typedef enum {} fmt_signed_char;
     FMT_NARG_(dummy, ##__VA_ARGS__, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, \
               4, 3, 2, 1, 0)
 
+// select between two expressions depending on whether __VA_ARGS__ is empty
+// expands to e if __VA_ARGS__ is empty and n otherwise
+#  define FMT_VA_SELECT(e, n, ...)                                            \
+    FMT_NARG_(dummy, ##__VA_ARGS__, n, n, n, n, n, n, n, n, n, n, n, n, n, n, \
+              n, n, e)
+
 #  define FMT_MAP_0(...)
 #  define FMT_MAP_1(f, a) f(a)
 #  define FMT_MAP_2(f, a, b) f(a), f(b)
@@ -168,11 +174,12 @@ typedef enum {} fmt_signed_char;
 #  define FMT_MAP(f, ...) \
     FMT_CAT(FMT_MAP_, FMT_NARG(__VA_ARGS__))(f, ##__VA_ARGS__)
 
-#  define fmt_format(buffer, size, fmt, ...)                              \
-    fmt_vformat(                                                          \
-        (buffer), (size), (fmt),                                          \
-        (fmt_arg[]){{fmt_int}, FMT_MAP(FMT_MAKE_ARG, ##__VA_ARGS__)} + 1, \
-        FMT_NARG(__VA_ARGS__))
+#  define fmt_format(buffer, size, fmt, ...)                                   \
+    fmt_vformat((buffer), (size), (fmt),                                       \
+                FMT_VA_SELECT(                                                 \
+                    NULL, ((fmt_arg[]){FMT_MAP(FMT_MAKE_ARG, ##__VA_ARGS__)}), \
+                    ##__VA_ARGS__),                                            \
+                FMT_NARG(__VA_ARGS__))
 
 #endif  // __cplusplus