encoding/asn1: class and tag constants shouldn't be private.

[perj]

I'm trying to extract an otherName from the subjectAltName of a certificate. Currently the code looks like this

if ext.Id.Equal(subjectAltName) {
    var altName asn1.RawValue
    asn1.Unmarshal(ext.Value, &altName)
    if altName.Class == 0 && altName.Tag == 16 {
        data := altName.Bytes
        for len(data) > 0 {
              var alt asn1.RawValue
              data, _ = asn1.Unmarshal(data, &alt)
              if alt.Class == 2 {
                  switch (alt.Tag) {
                  case 0:
                      ...
                  }
              }
          }
     }
}

As you can see there's a lot of magic numbers there. Almost all of those are defined constants in asn1/common.go, but they're set to private, so I can't use them. I don't think they should be private since they're well defined ASN.1 constants.

[rsc]

@agl for a decision

[bradfitz]

Considering that asn1.RawValue already exposes them, I'm also in favor of exporting the constants.

If they were public, it might be nice to make them typed constants so they can have methods, but https://golang.org/pkg/encoding/asn1/#RawValue already uses Tag and Class as just int, so I guess typed constants are out.

[bradfitz]

For @agl if he wants it: https://go-review.googlesource.com/17311

[gopherbot]

CL https://golang.org/cl/17311 mentions this issue.