Change:
return nil } <ins> st, dt := sv.Type(), dv.Type() </ins> switch st.Kind() { case reflect.String: if dt.Kind() == reflect.Interface && dv.NumMethod() == 0 {

Discussion:
Kubernetes version (use ): master. What happened: Unstructured converter from does not roundtrip . What you expected to happen: When I call on a which is backed by it should basically make a copy of the content (can be optimized btw using deep copy ) and not a json object with key. How to reproduce it (as minimally and precisely as possible): See the test in my branch Test output: Anything else we need to know: For some reason changed the argument types from to . I guess the intent is to make the converter a more generic tool. But based on the previous signature I assume I'm doing the right thing. Actually, my code works with and I don't know if the actual object behind the interface is an or not - I just pass it to the converter and it works for usual API types, but not for unstructured. Also it would be useful to have a more type-safe version of the converter (i.e. the previous signature) that just delegates to a lower-level converter that does not care about types.
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Looked into this issue just now. It is caused by the pointer vs non-pointer receiver of the method - similar to . In detail: is passed as to when it calls passing as . if passed value implements (has method ) does not implement this interface, does. Because on it . So perhaps the fix is to do the dereferencing inside of the ? Or maybe the same way as - check if the value is addressable, get a pointer to the value and then see if it matches the interface (I dunno if it will work that way)? In any case, the check whether a value implements the interface should be done on both the value itself () and the pointer to the value (; if the value is addressable) to cover both valid cases. This is what stdlib package does. Right now it is broken - it dereferences the pointer and then checks if the thing implements the interface. Does it make sense?

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does this helps? Look like we need to do MarshalJSON on non-pointers for it to work. I didn't get into the details but if that is the case then making Unstructured's MarshalJSON() receiver, a object instead of pointer should fix your problem, right?
Only if we don't fix the actual cause of the bug which I described above. From the stack overflow question that is mentioned in the link you provided: This code is buggy - to work it should be passing into and into . Yes, json package tries to take the address of the passed (so a pointer receiver should satisfy the interface check) but it cannot take an address of a function argument ( will return false). So, pointer receiver will be invoked if either is a field in a struct (so that an address can be taken via reflection) and a struct is passed (either by value or via a pointer) to / OR is passed. Please take a look at - both issues have the same cause.
I'm happy to take a stab at it and submit a PR but I don't want to spend my time if it is not going to get merged - if we could merge first to make sure people who can approve/lgtm are happy with the approach taken. I strongly believe it is the right thing to do - that is how standard library does it and, well, it is just logical and makes sense :)