`checker`: cannot pass a fixed array `[N]T` to a generic function expecting `[]T`

[enghitalo]

Describe the bug

A [N]T (fixed-size array) cannot be passed to a generic function that takes []T — V refuses to infer the inner element type, even though the relationship is mechanical ([N]T and []T share the element type).

This forces every generic decoder/encoder that wants a single code path for both array kinds to either copy via a sibling dynamic array (slow) or manually rewrite the dynamic-array header to alias the fixed-array storage (unsafe).

Reproduction Steps

fn fill[T](mut arr []T) {
	arr << T{}
}

fn fill_any[T](mut val T) {
	$if T is $array_fixed {
		fill(mut val)
	}
}

fn main() {
	mut fixed := [3]int{}
	fill_any(mut fixed)
	println(fixed)
}

Expected Behavior

Compiles. The checker infers T = int from [3]int matching []T.

Current Behavior

error: could not infer generic type `T` in call to `fill`
  10 |         fill(mut val)
     |         ~~~~~~~~~~~~~

Possible Solution

When inferring T for an mut []T parameter and the call-site argument has type [N]E, accept the inference T = E. The runtime can either coerce the fixed array into a []T view (length and cap = N, data = &fixed[0]) or generate a small adapter that copies in/out at the call boundary.

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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