cgen error: cannot convert 'struct string' to 'long'

[kbkpbot]

Describe the bug

generic T function may fail with strings.write_string but not strings.writeln.

found by PR #25857

Reproduction Steps

g.v

module main

import strings

pub struct MyStruct[T] {
pub mut:
        result T
        id     string
}

fn main() {
        x := MyStruct[int]{}
        y := MyStruct[string]{}

        dump(x.indent_str())
        dump(y.indent_str())
}

pub fn (it MyStruct[T]) indent_str[T]() string {
        mut res := strings.new_builder(128)
        res.write_string('${it.result}')
        // following is OK
        // res.writeln('${it.result}')
        return res.str()
}

v g.v

Expected Behavior

compile ok

Current Behavior

$ v g.v
================== C compilation error (from tcc): ==============
cc: /tmp/v_1000/g.01KBMWV9MNPR8A6ZW47790YW53.tmp.c:5033: error: cannot convert 'struct string' to 'long'
=================================================================
Try passing `-g` when compiling, to see a .v file:line information, that correlates more with the C error.
(Alternatively, pass `-show-c-output`, to print the full C error message).
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

use writeln instead of write_string, it will pass compile

Additional Information/Context

No response

V version

V 0.4.12 dafa6fd

Environment details (OS name and version, etc.)

V full version	V 0.4.12 dafa6fd
OS	linux, Ubuntu 24.04.3 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
Memory	6.54GB/15.51GB
V executable	/media/HD/github/kbkpbot/v/v
V last modified time	2025-12-04 14:35:47
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/autostr
Git version	git version 2.43.0
V git status	weekly.2025.46-135-gdafa6fd0
.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 2.39-0ubuntu8.6) 2.39

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