How to provide a dependency as itself and as an interface implementation?

[jp-dakota]

Take the following example, which will fail

package main

import "go.uber.org/fx"

type MyInterface interface {
	DoSomething()
}

type Obj struct{}

// DoSomething implements MyInterface.
func (*Obj) DoSomething() {
	// Whatever
}

func NewObj() *Obj {
	return &Obj{}
}

var _ MyInterface = (*Obj)(nil)

func main() {
	fx.New(
		fx.Provide(
			fx.Annotate(NewObj, fx.As(new(MyInterface))),
		),
		fx.Invoke(func(_ *Obj, _ MyInterface) {
		}),
	)
}

The above will fail with

[Fx] PROVIDE    main.MyInterface <= fx.Annotate(main.NewObj(), fx.As([[main.MyInterface]])
[Fx] PROVIDE    fx.Lifecycle <= go.uber.org/fx.New.func1()
[Fx] PROVIDE    fx.Shutdowner <= go.uber.org/fx.(*App).shutdowner-fm()
[Fx] PROVIDE    fx.DotGraph <= go.uber.org/fx.(*App).dotGraph-fm()
[Fx] INVOKE             main.main.func1()
[Fx] ERROR              fx.Invoke(main.main.func1()) called from:
main.main
        /workspace/tmp/dd.go:32
runtime.main
        /usr/local/go/src/runtime/proc.go:267
Failed: missing dependencies for function "main".main.func1
        /workspace/tmp/dd.go:33:
missing type:
        - *main.Obj (did you mean to use main.MyInterface?)

If I update main to

func main() {
	fx.New(
		fx.Provide(
			fx.Annotate(NewObj, fx.As(new(MyInterface))),
			NewObj,
		),
		fx.Invoke(func(_ *Obj, _ MyInterface) {
		}),
	)
}

then the constructor gets called twice.

Instantiating Obj outside fx and then using Supply would likely work in this simple example, but I wouldn't be able to leverage DI itself.

Is this even possible?
Thank you in advance!

[JacobOaks]

I think that fx.As can be used with multiple interface types, but since you want the struct and the interface, I don't think you can use one fx.As for both.

What you could do is add a separate "constructor" that returns *Obj as a MyInterface.

fx.Provide(
	NewObj,
	func(o *Obj) MyInterface {
		return o
	},
),
fx.Invoke(func(_ *Obj, _ MyInterface) {
}),