Mocks to handle 'nil' return values about mockery

[evanphx]

1. Could you give me a reproduction of a file that mockery generates an empty file for?
2. This is expected. You're hitting the weirdness around how golang treats interface{} and nil. Basically, you need to use m.On("getData").Return([]byte(nil), nil) in order to create a nil that is typed as []byte. If you pass a bare nil, it will end up is an interface{} with nil as the embedded type too. mockery could special case for []byte in the way you mention, but this is an issue for all types so I'd rather not have mockery hide this issue.

[mpalki1]

1. Here is the code. In my setup the Err object is actually in a different package.

package interfaces

type Err struct {
    msg  string
    code uint64
}

func (e *Err) Error() string {
    return e.msg
}

func (e *Err) Code() uint64 {
    return e.code
}

type KeyManager interface {
    GetKey(string, uint16) ([]byte, *Err)
}

1. Thanks for the tip on casting nil to []byte. However, this does not seem to work for custom interfaces that I have defined.
   For example if my getData returned an interface (called UserData) then when I say

m.On("getData").Return(interfaces.UserData(nil), nil)   //UserData is a custom interface that I have. 

then the line in the mock

r0 = args.Get(0).(interfaces.UserData)

panics saying "panic: interface conversion: interface is nil, not interfaces.UserData"

[evanphx]

Are you sure that error is coming from code mockery generated? It only uses the name ret, never args.

[mpalki1]

Yes, that error is from the code mockery generated. (I pasted my own mock above, which also has the same issue).
I therefore think it is not a mockery issue.

m.On("getData").Return(interfaces.UserData(nil), nil) does not work and panics. 
m.On("getData").Return((*UserDataImpl)(nil), nil)  works. UserDataImpl is a struct that implements the UserData interface. 

[evanphx]

Ah! Yes of course. You have to specify the concrete type because an interface value always references the concrete type, never an interface type. When you do interface.UserData(nil) it checks that a nil type and value can be a UserData and casts it to that interface only in the code. The value passed into the called function is still the nil type and value. The you cast to a concrete type, it emits a new value with type set as you'd want.

[mpalki1]

Thanks for the info about casting interface types. This conversation was quite helpful.