generic code struggles with optional function args

[spiveeworks]

Describe the bug

Calling generic higher order functions with optional callbacks leads to all kinds of C compile and runtime errors. I have tried to provide a concrete example that is easy to understand, but this gives issues completely different to the issues I was originally running into. This case still seem like a simpler place to start, before I try and build SSCCEs for the other issues I found.

Reproduction Steps

pub type MyFn[T] = fn (mut my_state T)

fn higher_order_fn[T](initial_state T, maybe_callback ?MyFn[T]) State {
    mut my_state := initial_state
    if callback := maybe_callback {
        callback[T](mut my_state)
        callback[T](mut my_state)
    }
    return my_state
}

pub struct State {
mut:
    x int
}

fn handler(mut my_state State) {
    my_state.x += 1
}

fn main() {
    mut state := State { }
    state = higher_order_fn[State](state, ?MyFn[State](handler))
    state = higher_order_fn[State](state, ?MyFn[State](none))
    state = higher_order_fn[State](state, handler)
    state = higher_order_fn[State](state, none)
    println("${state}")
}

Expected Behavior

I expect the above code to work, and to print State { x: 4 }. I also expect any subset of those four lines to work.

Current Behavior

Many things go wrong.

The first thing seems intentional:

higher_order_fn_error2.v:26:43: error: cannot use `none` as generic argument
   24 |     state = higher_order_fn[State](state, ?MyFn[State](none))
   25 |     state = higher_order_fn[State](state, handler)
   26 |     state = higher_order_fn[State](state, none)
      |                                           ~~~~
   27 |     println("${state}")
   28 | }

Fine, so let's remove line 26.

Now it compiles, but crashes:

Unhandled Exception 0xC0000005
C.CaptureStackBackTrace returned less than 2 frames
signal 11: segmentation fault
6 : print_backtrace_skipping_top_frames  ?? : address = 0x2a1510514d0
5 : seh_filter_exe             ?? : address = 0x2a1510515a0
4 : `__scrt_common_main_seh'::`1'::filt$0  D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:304
3 : _C_specific_handler        ?? : address = 0x2a151051690
2 : _chkstk                    ?? : address = 0x2a151051510
1 : RtlRaiseException          ?? : address = 0x2a1510516a0
0 : KiUserExceptionDispatcher  ?? : address = 0x2a151051580

so remove line 24, and run again:

State{
    x: 4
}

It works. Okay, let's remove line 23 now, and just call the function once.

higher_order_fn_error2.01HTVR8KKCEQBVVH81E32QQZ4E.tmp.c
C:\Users\Owner\AppData\Local\Temp\v_0\higher_order_fn_error2.01HTVR8KKCEQBVVH81E32QQZ4E.tmp.c(12786): error C2440: '=': cannot convert from '_option_anon_fn_mut_main__state' to '_option_anon_fn_mut_t'

builder error: msvc error

cgen seems to have screwed something up now. weird!

This may turn out to be one bug or two, but these were just the things I ran into this time. Other times I find these higher order functions work fine, and other times I think I run into other issues. I can experiment more and give more specific bug reports once the general chaos has been fixed a little.

Possible Solution

Good luck.

Additional Information/Context

Similar to #21214 I encountered these issues trying to make a wrapper for gg that handled streaming images automatically. Some apps provide event callbacks, other apps just provide do_frame callbacks with no event callbacks. I got it working, with lots of [T] annotations everywhere, but it was a minefield.

V version

V 0.4.5 ad17be5

Environment details (OS name and version, etc.)

V full version: V 0.4.5 386bd77.ad17be5
OS: windows, Microsoft Windows 10 Pro v19045 64-bit
Processor: 4 cpus, 64bit, little endian,

getwd: C:\Users\Owner\Code\v-experiments
vexe: C:\Program Files\v\v.exe
vexe mtime: 2024-04-07 05:22:56

vroot: contains spaces, value: C:\Program Files\v
VMODULES: OK, value: C:\Users\Owner.vmodules
VTMP: OK, value: C:\Users\Owner\AppData\Local\Temp\v_0

Git version: git version 2.37.2.windows.2
Git vroot status: weekly.2024.14-26-gad17be5d (6 commit(s) behind V master)
.git/config present: true

CC version: Error: 'cc' is not recognized as an internal or external command,
operable program or batch file.

thirdparty/tcc: N/A

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