cgen error when using sdl.Window as non-ref type for struct field

[larpon]

Describe the bug

================== C compilation error (from tcc): ==============
cc: /dev/shm/game.01JE1YPA1FWTHP3P6W2A0YMSBY.tmp.c:2549: error: field 'sdl_window' has incomplete type
=================================================================
(You can pass `-cg`, or `-show-c-output` as well, to print all the C error messages).

Reproduction Steps

This code with vlang/sdl 2.30.0 branch checked out in ~/.vmodules/sdl

import sdl

pub struct Window {
	sdl_window sdl.Window
}

fn main() {}

Expected Behavior

run + clean exit

Current Behavior

================== C compilation error (from tcc): ==============
cc: /dev/shm/game.01JE1Z9SGZT7BQ4S2NKTXZX233.tmp.c:2455: error: field 'sdl_window' has incomplete type
=================================================================
(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 .

Possible Solution

Some of this could maybe be prevented in vlang/sdl via attributes - but I think the C error should not happen.

Additional Information/Context

SDL2 v 2.30.0

V version

V 0.4.8 40dc775

Environment details (OS name and version, etc.)

EndeavourOS. Arch-based, rolling distro

Note

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_{Huly®: V_0.6-21482}

[larpon]

It has been discussed that maybe tagging such C constructs (opaque types) in V via @[opaque] and then giving checker error if it is not a pointer used could be a solution.

See discussion starting here: https://discord.com/channels/592103645835821068/592294828432424960/1312883921414914181