goverter:extend with interface does not seem to work as expected

[dropwhile]

Describe the bug goverter:extend with Interface does not seem to work as expected.

To Reproduce

Note: extracted from a larger codebase, so this example isn't quite exact

type RefID struct {
    SomeOpaqueThing // embedded type with a .String() method
}

type Notification struct {
    ID           int
    RefID        RefID
    Message      string
}

// protobuf generated
type PbNotification struct {
    RefId   string           
    Message string                
}

// goverter:converter
// goverter:matchIgnoreCase
// goverter:extend StringerToString
type Converter interface {
    ConvertNotifications(source []*Notification) []*PbNotification

    // goverter:ignoreUnexported
    ConvertNotification(source *Notification) *PbNotification
}

// I also tried (x fmt.Stringer)
func StringerToString(x interface{ String() string }) string {
    return x.String()
}

output

| *Notification
|
|     | Notification
|     |
|     | | RefID
|     | |
source*.RefID
target*.RefId
|     | |
|     | | string
|     |
|     | /PbNotification
|
| */PbNotification

TypeMismatch: Cannot convert RefID to string

Expected behavior I would have expected it to work.

Current work around I am using:

type Converter interface {
    ConvertNotifications(source []*Notification) []*PbNotification

    // goverter:ignoreUnexported
    // goverter:map . RefId | GetRefId
    ConvertNotification(source *Notification) *PbNotification
}

func GetRefId(x *Notification) string {
    return x.RefID.String()
}

It works, but then I have to annotate all methods (there will be many more instances/uses of RefID).

[jmattheis]

Goverter currently doesn't check if the source type is assignable to an interface of an extend method.

In the error message the conversion error occurs for the types RefID -> string, you could define the extend method like this.

// goverter:converter
// goverter:matchIgnoreCase
// goverter:extend StringerToString
type Converter interface {
    ConvertNotifications(source []*Notification) []*PbNotification

    // goverter:ignoreUnexported
    ConvertNotification(source *Notification) *PbNotification
}

func StringerToString(x RefID) string {
    return x.String()
}

This should be generic enough for this use-case. I'm not 100% against supporting interface assignment checks for extend methods, but it needs a better use-case for me to consider integrating it into goverter.

[dropwhile]

Unfortunately RefID in my example is one of many RefID "types" (used so static typing makes it harder to call methods with bad identifier types). I have a workaround for now, and I think my use case is probably a bit unusual or uncommon.

Thanks for the consideration though!