[c compilation bug] when using -autofree with json library

[AQMpolyface]

V version: V 0.4.9 f83af8a, press to see full `v doctor` output

V full version	V 0.4.9 f83af8a
OS	linux, "CachyOS"
Processor	16 cpus, 64bit, little endian, AMD Ryzen 7 Pro 7735U with Radeon Graphics
Memory	7.32GB/30.12GB
V executable	/home/polyface/git/v/v
V last modified time	2025-03-01 09:25:10
V home dir	OK, value: /home/polyface/git/v
VMODULES	OK, value: /home/polyface/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/polyface/Documents/v/bug
Git version	git version 2.48.1
V git status	weekly.2025.09-23-gf83af8a1
.git/config present	true
cc version	cc (GCC) 14.2.1 20250207
gcc version	gcc (GCC) 14.2.1 20250207
clang version	clang version 19.1.7
tcc version	tcc version 0.9.28rc 2024-07-31 HEAD@1cee0908 (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 0134e9b9
emcc version	N/A
glibc version	ldd (GNU libc) 2.41

What did you do?
./v -g -o vdbg cmd/v && ./vdbg -autofree src/main.v && src/main

// main.v
module main

import json

fn main() {
	input := '{"methode":"test"}'
	decode_message(input) or {}
}

pub fn decode_message(msg string) !string {
	decoded_json := json.decode(BaseMessage, msg.str())!
	return decoded_json.str()
}

type BaseMessage = struct {
	methode string @[json: methode]
}

What did you see?

================== C compilation error (from tcc): ==============
cc: /tmp/v_1000/main.01JN8GDSBDE2C9DZA5SNJJAHF4.tmp.c:6913: error: '_arg_expr_json_decode_1_177' undeclared
=================================================================
(You can pass `-cg`, or `-show-c-output` as well, to print all the C error messages).
builder error: 
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .

What did you expect to see?

A normal compilation. The bug only happens when passing -autofree to the compiler.

The bug seems to happen when you call .str() inside json.decode function. If you do something like this below, the bug doesn't happen

s := msg.str()
decoded_json := json.decode(BaseMessage, s)!

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[huly-for-github]

Connected to Huly®: V_0.6-22239

[AQMpolyface]

Also, I get a bug with this one. It seems like if there is any manipulation of the data inside json.decode while using autofree, it blows up.

pub fn decode_message(msg []u8) !(string, []u8) {
	json_content := msg.bytestr().split('\r\n\r\n')
	if json_content.len < 2 {
		return error("Erros: didnt received '\r\n\r\n'")
	}

	decoded_json := json.decode(BaseMessage, json_content[1])!
	return decoded_json.methode, json_content[1].bytes()
}

It is also fixed by using a variable instead of manipulating the data directly inside json.decode

pub fn decode_message(msg []u8) !(string, []u8) {
	json_content := msg.bytestr().split('\r\n\r\n')
	if json_content.len < 2 {
		return error("Erros: didnt received '\r\n\r\n'")
	}
        //note the new variable below
	to_decode := json_content[1]
	decoded_json := json.decode(BaseMessage, to_decode)!
	return decoded_json.methode, json_content[1].bytes()
}