#28308 suggests having vet report mistakes of the forms:

net.Dial("tcp", host+":"+port)
net.Dial("tcp", fmt.Sprintf("%s:%d", host, port))

The mistake being that if host is an IPv6 address then the address will be malformed.
The argument is supposed to be "[" + ipv6 + "]:" + port in that case,
and you should use

net.Dial("tcp", net.JoinHostPort(host, port))

instead.

SplitHostPort and JoinHostPort are important to use for URLs:
http://::1:80/ and http://::1/ are invalid, while http://[::1]:80/ and http://[::1]/ are fine.
(SplitHostPort and JoinHostPort only deal with the first of each pair.)

If you don't know whether a port is present, then the brackets resolve the ambiguity:
::1:80 is an IPv6 address, and [::1]:80 is an IPv6 address followed by a port.

But Dial with network "tcp" and "udp" does not have that "is the port present?" ambiguity: the port is required.
The port also never contains a colon.
So the final colon in the string is unambiguously the separator between host and port.

I wonder if we should make Dial with network "tcp" and "udp" accept host+":"+port.
Then all the things we're talking about flagging in #28308 would become correct instead.

The only downside is that if you mistakenly did

net.Dial("tcp", host)

(oops you forgot the port), then under certain rare circumstances that might not turn into a "invalid address error".
The vast majority of cases would behave as before:

• 👍 domain name like google.com → error as before
• 👍 ipv4 address like 192.168.0.1 → error as before
• 👍 ipv6 address like ::1 → error as before
• 👍 ipv6 address like 2001:db8::1 → error as before
• 👍 ipv6 address like 2001:db8::5a95 → error as before
• 👍 ipv6 address like 2001:db8::7226 → error as before
• 👍 ipv6 address like fe80::69aa:dcc:67ff:5a95 → error as before
• ✘ ipv6 address like fe80::e5e7:65b1:ff8a:7226 → dials [fe80::e5e7:65b1:ff8a]:7226

The only case that changes is when the IPv6 address ends in two or more non-wildcard chunks and the final chunk is entirely decimal digits. For the typical 16-bit chunks, that's only 15% of such addresses.

And of course if you do write this code, you're very likely to hit one of the error cases, and as soon as you do, you notice and fix the code.

On the other hand, consider

net.Dial("tcp", host+":"+port)

This works for domain names and IPv4 addresses and then breaks as soon as someone tries an IPv6 host.
We can make this work 100% of the time instead.

Overall, the cost of redefining that fairly unlikely mistake to be a different failure seems a small price to pay for fixing all the code that is using strings to put together Dial arguments and will fall over at the first IPv6 address literal.

Should we just fix all this code?