json.encode_pretty error when passing an inline initialized struct inside a function

[khalyomede]

Describe the bug

When passing an inline initialized struct to json.encode_pretty(), which is ran inside a function, it raises a C error.

Reproduction Steps

Run this code:

module main

import json

struct Definition {
    version u8
}

struct Logic {
    run fn () i8 @[required]
}

fn main() {
    logic := Logic{
        run: fn () i8 {
            json.encode_pretty(Definition{})

            return 0
        }
    }

    logic.run()
}

Or run it on the V playground

Expected Behavior

No error when formatting (pretty) a struct to JSON using an inline initialized struct.

Current Behavior

It raises this C error:

================== C compilation error (from gcc): ==============
cc: /tmp/v_0/main3.01JZXAN0TFJBFZ2DV2WMF20R8G.tmp.c: In function 'anon_fn_afedf1f6577f2f11___i8_165':
cc: /tmp/v_0/main3.01JZXAN0TFJBFZ2DV2WMF20R8G.tmp.c:2019:2: error: cannot convert to a pointer type
cc:  2019 |  cJSON* _t1 = json__encode_main__Definition((voidptr)((main__Definition){.version = 0,}));
cc:       |  ^~~~~
cc: /tmp/v_0/main3.01JZXAN0TFJBFZ2DV2WMF20R8G.tmp.c:2019:45: error: incompatible type for argument 1 of 'json__encode_main__Definition'
cc:  2019 |  cJSON* _t1 = json__encode_main__Definition((voidptr)((main__Definition){.version = 0,}));
cc:       |                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc:       |                                             |
cc:       |                                             void *
cc: /tmp/v_0/main3.01JZXAN0TFJBFZ2DV2WMF20R8G.tmp.c:2008:55: note: expected 'main__Definition' but argument is of type 'void *'
cc:  2008 | cJSON* json__encode_main__Definition(main__Definition val) {
cc:       |                                      ~~~~~~~~~~~~~~~~~^~~
=================================================================

Possible Solution

For the moment I countered this error by initializing the struct in a variable and passing the initialized variable to the function.

Additional Information/Context

Also note the same code will not raise a C error when the JSON pretty formatting is performed outside a function.

module main

import json

struct Definition {
	version u8
}

fn main() {
	json.encode_pretty(Definition{})
	
	println("Hello world!")
}

(or view it on the online V playground)

V version

V 0.4.11 d1d43ab

Environment details (OS name and version, etc.)

V full version	V 0.4.11 d1d43ab
OS	linux, N/A (VM)
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-10510U CPU @ 1.80GHz
Memory	2.98GB/3.68GB
V executable	/opt/v/v
V last modified time	2025-06-28 03:25:29
V home dir	OK, value: /opt/v
VMODULES	OK, value: /home/alpine/app/vine_packages
VTMP	OK, value: /tmp/v_0
Current working dir	OK, value: /home/alpine/app
env VFLAGS	"-cc gcc"
Git version	git version 2.30.6
V git status	d1d43ab
.git/config present	true
cc version	cc (Alpine 10.2.1_pre1) 10.2.1 20201203
gcc version	gcc (Alpine 10.2.1_pre1) 10.2.1 20201203
clang version	N/A
tcc version	tcc version 0.9.27 mob:d3e940c (x86_64 Linux)
tcc git status	thirdparty-linuxmusl-amd64 a3e24da2
emcc version	N/A
glibc version	Error: musl libc (x86_64)
Version 1.2.2
Dynamic Program Loader
Usage: /lib/ld-musl-x86_64.so.1 [options] [--] pathname

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[huly-for-github]

Connected to Huly®: V_0.6-23310

[jorgeluismireles]

Seems the problem is a combination of internal function calling json.encode_pretty. If is called with as an external function fn ext() i8 works. Though a simpler internal calling println('works') works too:

import json

struct Definition {
    version u8
}

struct Logic {
    run fn () i8 @[required]
}

fn ext() i8 {
	println(json.encode_pretty(Definition{}))
	return 0
}

fn main() {
    logic := Logic{
		run: ext // { "version":0 }
    }
    logic.run()
	other := Logic{
		run: fn() i8 {
			println('works')
			return 0
		}
	}
	other.run()
}

{
	"version":	0
}
works