cgen: fix C struct sumtype support

[felipensp]

Fix #21122

#include "@VMODROOT/foo.h"

struct C.Foo {
	a int
}

struct C.Bar {
	a int
}

type Baz = C.Foo | C.Bar

fn main() {
	mut a := Baz{}
	a = C.Foo{a: 1000}
	dump(a)	
}

[trufae]

ive tested this pr with the uing code and it builds but crashes. the resulting C code looks like this:

  uing__Window_set_child(w, HEAP(uing__Control, uing__Box_to_sumtype_uing__Control/*KEK*/(box)));

so it seems there's an extra abstraction for V sumtypes but it should just do a plain cast. Because i'm using a V struct as the public interface for those C.uiX structs which makes the public interface V-friendly while keeping the C structs internal to the module. So not sure if that's intended to be used that way

[felipensp]

ive tested this pr with the uing code and it builds but crashes. the resulting C code looks like this:

  uing__Window_set_child(w, HEAP(uing__Control, uing__Box_to_sumtype_uing__Control/*KEK*/(box)));

so it seems there's an extra abstraction for V sumtypes but it should just do a plain cast. Because i'm using a V struct as the public interface for those C.uiX structs which makes the public interface V-friendly while keeping the C structs internal to the module. So not sure if that's intended to be used that way

Yes, thanks for testing. I'll need to change a bit the sumtype implementation after the major PR about sumtype get merged.

[trufae]

still seems to not make my code to work, sumtypes contain the data and the type, while i think for those cases it should just pass the data :? but at least its getting closer. also another issue is that in my case i want Control to be a sumtype for Box, Label, Button.. but at the same time all those Box Label Button types should be castable to C.uiControl. so there's an extra level of indirection in my case..

[spytheman]

Very good work.