Implementing an Interface with a Generic Method Receiver

March 31, 2022

I’m trying to implement an interface while allowing generics. The simplified scenario looks like this:

type ID[T uuid.UUID | string] struct {
    ...
}

var _ encoding.TextUnmarshaler = (*ID)(nil)

func (id *ID[T]) UnmarshalText(data []byte) error {
    ...
}

I end up with the error cannot use generic type ID without instantiation for the var _ ... line when I run the code, and my IDE tells me that ID is missing the required method.
Dropping the [T] in the method receiver head isn’t allowed either because of this instantiation issue.

How can I proceed from here?

>Solution :

You can do:

var _ encoding.TextUnmarshaler = (*ID[string])(nil)

https://go.dev/play/p/WmlJ0JEKeVp

Whenever you get the cannot use generic type ... without instantiation error, then that means that the compiler expects you to explicitly provide the type argument.

Instantiations: