ErrorUnset conflicts with UntaggedFields

[fedorkanin]

Issue mitchellh#350 in the original repo by mateusoliveira43 should be addressed:

If I use ErrorUnset = true and add mapstructure:"-" to my struct field, I get the following error:

* '' has unset fields: -

But since that field is ignored, it should not trigger an error, right?

Also tried using with IgnoreUntaggedFields = true, but got the same result

Current workaround is to check the mapstructure.Metadata manually. Minimal example:

md := mapstructure.Metadata{}
opts := func(mcfg *mapstructure.DecoderConfig) {
	mcfg.Metadata = &md
}
viper.Unmarshal(&cfg, opts)

// Ignore unset fields that have the "-" mapstruct tag.
md.Unset = slices.DeleteFunc(md.Unset, func(s string) bool {
	return strings.HasSuffix(s, ".-")
})

require.Empty(t, md.Unused, "unused config keys: %v", md.Unused)

Here is also a playground example, however it's build times out, playground has limited resources.

If this is by design? Any clarification is appreciated!