fix

vlib/v/checker/fn.v

@@ -1593,7 +1593,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 			|| param.typ == ast.nil_type || final_param_sym.idx == ast.nil_type_idx)
 			&& !call_arg.typ.is_any_kind_of_pointer() && func.language == .v
 			&& !call_arg.expr.is_lvalue() && !c.pref.translated && !c.file.is_translated
-			&& func.name !in ['json.encode', 'json.encode_pretty'] {
+			&& !func.is_c_variadic && func.name !in ['json.encode', 'json.encode_pretty'] {
 			c.error('expression cannot be passed as `voidptr`', call_arg.expr.pos())
 		}
 		// Handle expected interface

vlib/v/gen/c/testdata/literal_c_variadic_arg.out

@@ -0,0 +1 @@
+foo : 1 : 2.50, 42, v

vlib/v/gen/c/testdata/literal_c_variadic_arg.vv

@@ -0,0 +1,22 @@
+#include <stdarg.h>
+
+@[typedef]
+struct C.va_list {}
+
+fn C.va_start(voidptr, voidptr)
+fn C.va_end(voidptr)
+
+fn C.vfprintf(&C.FILE, &char, C.va_list) int
+
+fn t2(fmt voidptr, ...) {
+	ap := C.va_list{}
+	C.va_start(ap, fmt)
+	C.vfprintf(C.stderr, fmt, ap)
+	C.va_end(ap)
+}
+
+fn main() {
+	a := 1
+	b := 2.5
+	t2(c'%s : %d : %.2f, %d, %c\n', c'foo', a, b, 42, `v`)
+}