cgen erro: wrong sumtype cast function?

[enghitalo]

Describe the bug

Reproduction Steps

struct StructType[T] {
mut:
	val T
}

type SumTypes = StructType[string] | []SumTypes | []string | bool | int | string

pub struct Count {
mut:
	total int
}

// count_chars count json sizen whithout new encode
pub fn (mut count Count) count_chars[T](val T) {
	$if T is $sumtype {
		$for v in val.variants {
			if val is v {
				// dump(typeof(val).name)
				count.count_chars(val)
			}
		}
	} $else $if T is $struct {
		count.chars_in_struct(val)
	} $else {
	}
}

// chars_in_struct
fn (mut count Count) chars_in_struct[T](val T) {
	$for field in T.fields {
		va := val.$(field.name)
		count.count_chars(va)  // Not works
		count.count_chars(val.$(field.name)) // Not works
	}
}

fn main() {
	mut count := Count{}
	count.count_chars(StructType[SumTypes]{ val: '' })
	println('ok')
	assert true
}

Expected Behavior

assert pass, print ok

Current Behavior

==================
/tmp/v_1000/a.01HRM75XMWA9WKHPFBFAQVD0EY.tmp.c:13325: error: cannot convert 'struct main__SumTypes' to 'struct string'
...
==================
(Use `v -cg` to print the entire error message)

builder error: 
==================
C error. This should never happen.

This is a compiler bug, please report it using `v bug file.v`.

https://github.com/vlang/v/issues/new/choose

You can also use #help on Discord: https://discord.gg/vlang

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.4 386bd77

Environment details (OS name and version, etc.)

Linux

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