cgen: V auto functions is not working in Sumtype variants declaration

[enghitalo]

Describe the bug

cgen Error

Reproduction Steps

module main

struct Chacabum {
	a int
}

type SumType = Chacabum | int | string


fn (v SumType) cast[T](val T) string {

	mut res := 'lala'

	$for variant_value in v.variants {
		if v is variant_value {
			// println(v) // ok
			// println('v: ${v.str()}') // ok
			// println('v: ${v}') // not ok <-----------------------------------------------------------------
			// return v // ok
			// res = v.str() // ok
			// res = 'v: ${v.str()}' // ok
			res = 'v: ${v}' // not ok <-----------------------------------------------------------------
		}
	}

	return res

}

fn main() {
	assert SumType(1).cast[int](1) == 'v: 1'
}

Expected Behavior

assert SumType(1).cast[int](1) == 'v: 1' pass

Current Behavior

================== C compilation error (from tcc): ==============
cc: /tmp/v_1000/call_methods_at_comptime2.01J91SAWTKJPEVTEFH0X1S8VT4.tmp.c:13382: error: cannot convert 'struct main__Chacabum' to 'unsigned char *'
=================================================================
(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

make res = 'v: ${v}' works as res = 'v: ${v.str()}'?

Additional Information/Context

No response

V version

V 0.4.8 aa3ab3f

Environment details (OS name and version, etc.)

Ubuntu

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