Unexpected behavior unmarshaling into interface field

[rickbau5]

I am trying to use an interface{} field on a struct to unmarshal into arbitrary struct types using a single field. I'd like to know if the behavior i'm seeing is expected or if there is a different way to accomplish this. I've linked to an example on the Go Playground but will include a few excerpts.

I have a few structs, for example:

type Envelope struct {
	Data interface{} `yaml:"data"`
}

type Foo struct {
	Greeting string `yaml:"greeting"`
}

type Bar struct {
	Farewell string `yaml:"farewell"`
}

And I'm trying to support these configs, as an example:

---
data:
  greeting: "Hello from YAML"`

---
data:
  farewell: "Goodbye from YAML"

I would expect to be able to unmarshal one of those configs using an Envelope with a pointer to Foo or Bar resulting in the fields on Foo or Bar being set to the values in the configuration. Instead, Data on Envelope is a map[string]interface{}.

For example:

var foo Foo
envelope := Envelope{
	Data: &foo,
}

err := yaml.Unmarshal([]byte(yamlPayload), &envelope)
if err != nil {
	panic(err)
}

fmt.Printf("%#v\n", envelope) // main.Envelope{Data:map[string]interface {}{"greeting":"Hello from YAML"}}
fmt.Printf("%#v\n", envelope.Data) // map[string]interface {}{"greeting":"Hello from YAML"}

I would have expected this to print the following:

main.Envelope{Data:(*main.Foo)(0xc000010280)}
&main.Foo{Greeting:"Hello from YAML"}

The unmarshalling doesn't respect the point to the struct set on the interface field and overwrites it with map[string]interface{}

The playground link shows a complete example and includes an example with this behavior from the encdoing/json Unmarshal function, which gives the desired results.
Playground Link