Replace the dummy arg from compound initializers in the C API with more macro magic (#4696)

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.

In order to avoid a quadratic number of expansions, defer creating the
mapped argument list until after selection of whether to use NULL or
the fmt_arg array.

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

Author: Ferdi265

include/fmt/fmt-c.h

@@ -130,7 +130,7 @@ typedef enum {} fmt_signed_char;
 #  define FMT_NARG_(_id, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, \
                     _13, _14, _15, _16, N, ...)                             \
     N
-#  define FMT_NARG(...)                                                        \
+#  define FMT_NARG(_dummy, ...)                                                \
     FMT_NARG_(dummy, ##__VA_ARGS__, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, \
               4, 3, 2, 1, 0)
 
@@ -166,13 +166,24 @@ typedef enum {} fmt_signed_char;
         f(n), f(o), f(p), f(q)
 
 #  define FMT_MAP(f, ...) \
-    FMT_CAT(FMT_MAP_, FMT_NARG(__VA_ARGS__))(f, ##__VA_ARGS__)
+    FMT_CAT(FMT_MAP_, FMT_NARG(, ##__VA_ARGS__))(f, ##__VA_ARGS__)
+
+// 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_MAKE_NULL(...) NULL
+#  define FMT_MAKE_ARGLIST(...) \
+    (fmt_arg[]) { FMT_MAP(FMT_MAKE_ARG, ##__VA_ARGS__) }
+#  define FMT_EXPAND(v) v
 
 #  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__))
+    fmt_vformat((buffer), (size), (fmt),                                  \
+                FMT_EXPAND(FMT_VA_SELECT(FMT_MAKE_NULL, FMT_MAKE_ARGLIST, \
+                                         ##__VA_ARGS__)(__VA_ARGS__)),    \
+                FMT_NARG(, ##__VA_ARGS__))
 
 #endif  // __cplusplus