What version of Go are you using (go version)?

1.7.3

What operating system and processor architecture are you using (go env)?

any

Enhancement

Since byName sorts by index depth and tag presence, the dominant field is always the first. So the dominantField function can be simplified to checking for the error case: more than one field at top level, both tagged or both untagged.

func dominantField(fs []field) (field, bool) {
    if len(fs) > 1 && len(fs[0].index) == len(fs[1].index) && fs[0].tag == fs[1].tag {
        return field{}, false
    }
    return fs[0], true
}

See https://code-review.googlesource.com/9552.