assigning to mutable optional give v error: `next` is immutable, declare it with `mut` to make it mutable

[daansystems]

V version: V 0.4.10 880f216, press to see full `v doctor` output

V full version	V 0.4.10 5c65e58.880f216
OS	windows, Microsoft Windows 11 Pro 26100 64-bit
Processor	24 cpus, 64bit, little endian, AMD Ryzen 9 3900X 12-Core Processor
Memory	18.12GB/31.9GB
V executable	C:\v\v.exe
V last modified time	2025-04-28 07:34:43
V home dir	OK, value: C:\v
VMODULES	OK, value: C:\Users\info.vmodules
VTMP	OK, value: C:\Users\info\AppData\Local\Temp\v_0
Current working dir	OK, value: D:\vbug2
Git version	git version 2.42.0.windows.2
V git status	weekly.2025.17-12-g880f2169-dirty
.git/config present	true
cc version	N/A
gcc version	N/A
clang version	clang version 17.0.1
msvc version	N/A
tcc version	tcc version 0.9.27 (x86_64 Windows)
tcc git status	thirdparty-windows-amd64 b425ac82
emcc version	N/A
glibc version	ldd (cygwin) 3.4.7

What did you do?
./v -g -o vdbg cmd/v && ./vdbg main.v && D:\vbug2\main.exe

module main

@[heap]
interface IGameObject {
mut:
	name       string
	parent     ?&IGameObject
	next       ?&IGameObject
	child      ?&IGameObject
	last_child ?&IGameObject
}

@[heap]
struct GameObject implements IGameObject {
mut:
	name       string
	parent     ?&IGameObject
	next       ?&IGameObject
	child      ?&IGameObject
	last_child ?&IGameObject
}

fn main() {
	mut v1 := &GameObject{
		name: 'v1'
	}

	mut next := v1.next
	for {
		if next != none {
			eprintln(next.name)
			next = next.next
		} else {
			break
		}
	}
}

What did you see?

main.v:32:4: error: `next` is immutable, declare it with `mut` to make it mutable
   30 |         if next != none {
   31 |             eprintln(next.name)
   32 |             next = next.next
      |             ~~~~
   33 |         } else {
   34 |             break

What did you expect to see?

no v error

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[huly-for-github]

Connected to Huly®: V_0.6-22727

[jorgeluismireles]

Seems next mutability is lost after unwrapped as option. I think this could be a workaround to traverse the linked list:

interface IGameObject {
mut:
	next ?IGameObject
}

struct GameObject implements IGameObject {
mut:
	next ?IGameObject
}

fn main() {
	mut o2 := GameObject{}
	mut o1 := GameObject{ next:o2 }
	mut obj := IGameObject(o1)
	for {
		if obj.next != none { println('continue')
			obj = obj.next
		} else { println('stop')
			break
		}
	}
	println('end')
}

continue
stop
end

[felipensp]

You should do if mut next != none { but it was generating wrong code when I tried... So I've fixed cgen generated code.