cgen: fix compiling call expressions with no varargs (tcc bug)

vlib/v/gen/cheaders.v

@@ -3,7 +3,6 @@ module gen
 // NB: @@@ here serve as placeholders.
 // They will be replaced with correct strings
 // for each constant, during C code generation.
-
 const (
 	// V_COMMIT_HASH is generated by cmd/tools/gen_vc.v .
 	c_commit_hash_default         = '
@@ -17,8 +16,8 @@ const (
 	#define V_CURRENT_COMMIT_HASH "@@@"
 #endif
 '
-
 	c_common_macros               = '
+#define EMPTY_VARG_INITIALIZATION 0
 #define EMPTY_STRUCT_DECLARATION
 #define EMPTY_STRUCT_INITIALIZATION 0
 // Due to a tcc bug, the length of an array needs to be specified, but GCC crashes if it is...
@@ -53,8 +52,10 @@ const (
 #endif
 
 #ifdef __TINYC__
+	#undef EMPTY_VARG_INITIALIZATION
 	#undef EMPTY_STRUCT_DECLARATION
 	#undef EMPTY_STRUCT_INITIALIZATION
+	#define EMPTY_VARG_INITIALIZATION
 	#define EMPTY_STRUCT_DECLARATION char _dummy
 	#define EMPTY_STRUCT_INITIALIZATION 0
 	#undef EMPTY_ARRAY_OF_ELEMS
@@ -75,8 +76,7 @@ const (
 
 // for __offset_of
 #ifndef __offsetof
-	#define __offsetof(s,memb) \\
+	#define __offsetof(s,memb) ((size_t)((char *)&((s *)0)->memb - (char *)0))
-	((size_t)((char *)&((s *)0)->memb - (char *)0))
 #endif
 
 #define OPTION_CAST(x) (x)

vlib/v/gen/fn.v

@@ -836,7 +836,8 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 				}
 			}
 		} else {
-			g.write('0')
+			// NB: tcc can not handle 0 here, while msvc needs it
+			g.write('EMPTY_VARG_INITIALIZATION')
 		}
 		g.write('}}')
 	}

vlib/v/tests/vargs_empty_param_test.v

@@ -0,0 +1,20 @@
+struct Config {
+	token string
+}
+
+struct Client {
+	x u64
+	y u64
+}
+
+fn new(config Config, shard_count ...int) ?&Client {
+	return &Client{1, 2}
+}
+
+fn test_can_compile_an_empty_var_arg() {
+	x := new(Config{
+		token: 'xyz'
+	}) or { panic(err) }
+	assert x.x == 1
+	assert x.y == 2
+}