json: fix json.decode autofree codegen (fix #23834) (#23839)

vlib/v/gen/c/fn.v

@@ -2477,9 +2477,10 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 			}
 		} else {
 			if use_tmp_var_autofree {
+				n := if node.name == 'json.decode' { i + 2 } else { i + 1 }
 				// TODO: copypasta, move to an inline fn
 				fn_name := node.name.replace('.', '_')
-				name := '_arg_expr_${fn_name}_${i + 1}_${node.pos.pos}'
+				name := '_arg_expr_${fn_name}_${n}_${node.pos.pos}'
 				g.write('/*af arg2*/' + name)
 			} else {
 				g.expr(arg.expr)

vlib/v/gen/c/testdata/autofree_json_decode.c.must_have

@@ -0,0 +1,2 @@
+string _arg_expr_json_decode_2_271 = string_str(msg);
+cJSON* _t2 = json__json_parse(/*af arg2*/_arg_expr_json_decode_2_271);

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

@@ -0,0 +1,19 @@
+// vtest vflags: -autofree
+module main
+
+import json
+
+fn main() {
+	input := '{"methode":"test"}'
+	decode_message(input) or {}
+	assert true
+}
+
+type BaseMessage = struct {
+	methode string @[json: methode]
+}
+
+pub fn decode_message(msg string) !string {
+	decoded_json := json.decode(BaseMessage, msg.str())!
+	return decoded_json.str()
+}