Too late for Go 1.5.

As of Go1.5, I don't see any logic in the Reader that explicitly forbids "/" path. In fact, it seems that the Reader does properly read back "/". See playground

The "standards" doesn't explicitly reject "/". Personally, I think that tar should not forbid any paths. If "/" is rejected, should we also reject ""? What about absolute paths? As it is, any proper user of tar should be sanitizing the paths read out anyways.

Furthermore, I can imagine a possible use case of "/" where it is encoded into an archive to preserve uid, gid, or something similar.

Closing this. Although discouraged (and the tar utilities will warn), it is possible to have a folder named "/" and to have owner and permission bits set in the tar file and expect the tar utility to set the root directory to those values. I don't think we should forbid that behavior.