`$zero(field.typ.payload_type)` inside `$for field in T.fields` uses the same payload type for every option field

[enghitalo]

Describe the bug

Inside $for field in T.fields, when a struct has multiple ?T fields with different non-pointer payload types, $zero(field.typ.payload_type) resolves to the same payload type for every iteration (the last one) instead of the current field's payload type. This causes a C compilation error.

Introduced by 8b5f74e.

Related: #26995 — same family of bug for T.pointee_type.

Reproduction Steps

module main

struct Foo {
mut:
    a ?int
    b ?string
}

fn fill[T](mut x T) {
    $for field in T.fields {
        $if field.typ is $option {
            mut v := $zero(field.typ.payload_type)
            x.$(field.name) = v
        }
    }
}

fn main() {
    mut f := Foo{}
    fill(mut f)
    dump(f)
}

Expected Behavior

Compile cleanly. f.a becomes Some(0) and f.b becomes Some('').

Current Behavior

================== C compilation error (from tcc): ==============
cc: /tmp/v_1000/<...>.tmp.c:8530: error: cannot convert 'struct string' to 'int'
=================================================================

With gcc:

error: incompatible types when initializing type 'int' using type 'string'
 8530 |    builtin___option_ok(&(int[]) { v }, (_option*)(&_t2), sizeof(int));

Generated C for the a: ?int iteration declares v as string (the payload type of b: ?string, the last option field) and then tries to wrap it in _option_int, hence the type mismatch.

Possible Solution

Resolve field.typ.payload_type per-iteration of $for field in T.fields instead of using a single resolved type for the whole expansion.

V version

V 0.5.1 6dd9033.eb1d47b

Environment details

linux, Ubuntu 24.04 LTS, gcc 14.2.0, tcc 0.9.28rc

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.