by mt4swm:

What steps will reproduce the problem?

See http://play.golang.org/p/gvAUHRN19-

What is the expected output?

I would expect, as the result of net.SplitHostPort("[de:ad:be:ef::ca:fe]"),
the error message "missing port in address ...".

What do you see instead?

"too many colons in address ..."

Which compiler are you using (5g, 6g, 8g, gccgo)?
Which operating system are you using?
Which version are you using?  (run 'go version')
8g, Linux, +835576f9bf0e

Please provide any additional information below.

The modified function below changes the behaviour. After the
test for brackets around host fails, the function also tests
for brackets around hostport now.

func splitHostPort(hostport string) (host, port, zone string, err error) {
    // The port starts after the last colon.
    i := last(hostport, ':')
    if i < 0 {
        err = &AddrError{"missing port in address", hostport}
        return
    }
    host, port = hostport[:i], hostport[i+1:]
    // Can put brackets around host ...
    switch {
    case insideBrackets(host):
        host = host[1 : len(host)-1]
    case insideBrackets(hostport):
        err = &AddrError{"missing port in address", hostport}
    default:
        // ... but if there are no brackets, no colons.
        if byteIndex(host, ':') >= 0 {
            err = &AddrError{"too many colons in address", hostport}
            return
        }
    }
    return
}

func insideBrackets(s string) bool {
    return len(s) > 0 && s[0] == '[' && s[len(s)-1] == ']'
}