Fix merging of interface types with concrete values

[admtnnr]

No description provided.

[vdemeester]

@admtnnr Thanks for the PR. Can you describe a bit more why is this needed (which use cases, …) ?

[admtnnr]

@vdemeester sure, here's the case I ran into:
Say we have an interface:

type Doer interface {
  Do() error
}

And we have two implementations of that interface:

type NoopDoer string
func (d NoopDoer) Do() error { return nil }

type RealDoer struct { ... }
func (d RealDoer) Do() error {
  ...
  return nil
}

And we have our config struct like so:

type config struct {
   Doer Doer 
}

And we set up a "default" config that we will merge in that uses the NoopDoer we defined above:

var defaultConfig = config{
  Doer: NoopDoer("this does nothing"),
}

What I've encountered is that the call to mergo.Merge fails if you have a setup where you pass in a config that also has a Doer defined that should take precedence, but is a different concrete type.
So something like this will return an error and panic where it shouldn't since the config should accept any implementation of a Doer:

var myConfig = config{
  Doer: RealDoer{},
}
if err := mergo.Merge(&myConfig, defaultConfig); err != nil {
   panic("THIS WILL GET RUN")
}

So what actually seems to happen is that mergo doesn't check that the types are actually assignable to the field in the struct, but rather just checks that the source and destination types are the same.

It skips through this case since the underlying types aren't the same: https://github.com/imdario/mergo/pull/106/files#diff-a103b5fee5fb31abebf12ee067c01da8R190 and instead hits this line: https://github.com/imdario/mergo/pull/106/files#diff-a103b5fee5fb31abebf12ee067c01da8R195

The test I added will fail if the check for AssignableTo is removed even though both types implement http.Handler.

Hopefully that helps explain it a little better. It's a weird case, I'll give you that. :)

[vdemeester]

So what actually seems to happen is that mergo doesn't check that the types are actually assignable to the field in the struct, but rather just checks that the source and destination types are the same.

@admtnnr indeed 😅

[vdemeester]

@admtnnr btw can you rebase against master to fix the CI ? 👼

[admtnnr]

@vdemeester Rebased.

[admtnnr]

@vdemeester let me know if there's anything else you need from me to get this merged. :)

[vdemeester]

LGTM :tiger: