mut sumtype value can't be changed

[kbkpbot]

Describe the bug

When a function try to change a mut sumtype, it will fail.

Reproduction Steps

mut.v

module main

struct CondA {
pub mut:
        val int
}

struct CondB {
pub mut:
        val int
}

type Cond = CondA | CondB

fn func(mut cond Cond) {
        if mut cond is CondA {
                cond.val = 1234
        }
}

fn main() {
        mut x := CondA{}
        x.val = 1

        func(mut x)
        assert x.val == 1234
        dump(x)
}

v mut.v

run it

Expected Behavior

run without assert fail

Current Behavior

$ ./mut
mut.v:26: FAIL: fn main.main: assert x.val == 1234
   left value: x.val = 1
  right value: 1234
V panic: Assertion failed...
v hash: 7d67e22
/tmp/v_1000/mut.01K2MD1A9PM22ZM47G85ADYHQB.tmp.c:3691: at _v_panic: Backtrace
/tmp/v_1000/mut.01K2MD1A9PM22ZM47G85ADYHQB.tmp.c:5352: by main__main
/tmp/v_1000/mut.01K2MD1A9PM22ZM47G85ADYHQB.tmp.c:5381: by main

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.11 d5458b5.7d67e22

Environment details (OS name and version, etc.)

V full version	V 0.4.11 d5458b5.7d67e22
OS	linux, Ubuntu 24.04.3 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
Memory	1.24GB/15.51GB
V executable	/media/HD/github/kbkpbot/v/v
V last modified time	2025-08-14 13:31:04
V home dir	OK, value: /media/HD/github/kbkpbot/v
VMODULES	OK, value: /home/mars/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/mars/v/bug/mut
Git version	git version 2.43.0
V git status	weekly.2025.17-547-g7d67e22b-dirty
.git/config present	true
cc version	cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
gcc version	gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
clang version	Ubuntu clang version 18.1.3 (1ubuntu1)
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.39-0ubuntu8.5) 2.39

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[huly-for-github]

Connected to Huly®: V_0.6-23631

[felipensp]

$ v -d mutable_sumtype=true -g self
$ v run bug.v
[bug.v:27] x: CondA{
    val: 1234
}

[Delta456]

Why is this with a flag?

[felipensp]

When I fixed this it can't be applied to V directly because the v.c can't handle it. It's about V bootstrapping phase.

[Delta456]

Is there some other way to do it? If not, we should have a message to enable this when this feature is used.

[JalonSolov]

It will likely not need a special switch once the bootstrapping is complete, so no need to worry about special messages, etc. since this is an edge case that's only been reported once.

[kbkpbot]

Original code only setup a field _typ.
But as sumtype has common fields, so they are also need to setup.

// Union sum type v__ast__TypeDecl =
//          |  332 = v__ast__AliasTypeDecl
//          |  333 = v__ast__FnTypeDecl
//          |  334 = v__ast__SumTypeDecl
struct v__ast__TypeDecl {
        union {
                v__ast__AliasTypeDecl* _v__ast__AliasTypeDecl;
                v__ast__FnTypeDecl* _v__ast__FnTypeDecl;
                v__ast__SumTypeDecl* _v__ast__SumTypeDecl;
        };
        int _typ;
        string* name;                         // all these fields need to setup ....
        bool* is_pub;
        v__ast__Type* typ;
        v__token__Pos* pos;
        Array_v__ast__Attr* attrs;
        bool* is_markused;
};

currently I fixed the code, but it stuck at -usecache....
11f6770