v: smartcast comptime variable

[felipensp]

Fix #20176

type TestSum = int | string
struct Abc { s TestSum }

fn gen[T,R](struc T) R {
    $if T is $struct {
        $for field in T.fields {
            field_value := struc.$(field.name)
            $if field_value is $sumtype {
                $for v in field_value.variants {
                    if field_value is v { //smartcast
                        $if field_value is R {
                            dump(field_value)
                            return field_value
                        }
                    }
                }
            }
        }
    }
    return R{}
}

fn main() {
    a := Abc{TestSum(123)}
    b := Abc{TestSum('foo')}
    int_var := gen[Abc,int](a)
    str_var := gen[Abc,string](b)
    assert dump(str_var) == 'foo'
    assert dump(int_var) == 123
}

[spytheman]

comment/remove this?

[spytheman]

Excellent work!