`cgen`: comptime `$for f in T.fields` over a generic struct with `?[]T` / `?[N]T` fields generates invalid C

[enghitalo]

Describe the bug

A generic function that walks the fields of a generic struct containing ?T, ?[]T and ?[N]T fields (where T is the generic parameter) generates invalid C: the codegen emits _option_string and _option_Array_string initializers as if they were raw byte arrays, producing both cannot convert 'struct _option_string' to 'unsigned char *' and incompatible types when initializing 'u8 *' using '_option_Array_string' errors.

Reproduction Steps

pub struct Filter[T] {
pub mut:
	selector  ?T
	selector2 ?[]T
	selector3 ?[1]T
}

fn process_field[T](val T) {
	_ = val
}

fn process_struct[T](val T) {
	$for f in T.fields {
		process_field(val.$(f.name))
	}
}

fn main() {
	t := Filter[string]{
		selector:  'aa'
		selector2: ['a', 'b']
		selector3: ['z']!
	}
	process_struct(t)
}

Expected Behavior

Compiles. The comptime walker visits each field; the per-field generic gets instantiated for ?string, ?[]string, ?[1]string respectively.

Current Behavior

================== C compilation error ==============
error: cannot convert 'struct _option_string' to 'unsigned char *'
        builtin___option_ok(&(Array_fixed_string_1[]) {val }, (_option*)(&_t1), sizeof(Array_fixed_string_1));
                                                       ^~~
error: incompatible types when initializing type 'u8 *' using type '_option_Array_string'
        builtin___option_ok(&(Array_fixed_string_1[]) {val }, (_option*)(&_t1), sizeof(Array_fixed_string_1));
                                                       ^~~
=====================================================
builder error: C error found while compiling generated C code.

Possible Solution

When the comptime $for f in T.fields walker over a generic struct sees field.typ of ?T, ?[]T or ?[N]T, the option's payload-init codegen drops the array layer somewhere on the way from field.typ resolution to the _option_X C struct emission. The result is that _option_string gets initialized through the path that expects an [N]T payload, leading to Array_fixed_string_1[] being assigned a struct value.

The fix is in the comptime/cgen layer that monomorphizes generic structs with option-of-array fields: produce a payload struct whose layout matches the option's payload type, not the surrounding fixed-array shape.

V version

V 0.5.1 (40b3711)

Environment details (OS name and version, etc.)

|V full version      |V 0.5.1 1b3385cc34ff783e793d1a26a8ec5be587c80fe0.40b3711
|:-------------------|:-------------------
|OS                  |linux, Ubuntu 24.04 LTS
|Processor           |16 cpus, 64bit, little endian, AMD Ryzen 7 5800H with Radeon Graphics
|Memory              |10.02GB/30.7GB
|                    |
|V executable        |/home/hitalo/Documents/v/v
|V last modified time|2026-04-18 09:18:00
|                    |
|V home dir          |OK, value: /home/hitalo/Documents/v
|VMODULES            |OK, value: /home/hitalo/.vmodules
|VTMP                |OK, value: /tmp/v_1000
|Current working dir |OK, value: /home/hitalo/Documents/v
|                    |
|Git version         |git version 2.43.0
|V git status        |0.5.1-1006-g40b3711b-dirty
|.git/config present |true
|                    |
|cc version          |cc (GCC) 14.2.0
|gcc version         |gcc (GCC) 14.2.0
|clang version       |Ubuntu clang version 18.1.3 (1)
|tcc version         |tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
|tcc git status      |thirdparty-linux-amd64 696c1d84
|emcc version        |emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.6 ()
|glibc version       |ldd (Ubuntu GLIBC 2.39-0ubuntu8.3) 2.39

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