cmd/compile: missing hint to check go.mod if language version < 1.18 and declare a generic type or func

[thepudds]

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

Tip:

go version devel go1.19-63bd6f68e6 Mon Mar 7 15:02:48 2022

Does this issue reproduce with the latest release?

Yes.

What did you see?

I happened to notice a very experienced gopher this morning struggling a bit to figure out why they couldn't use generics:

I'm experimenting with generics again, with the release candidate. But now I see "type parameters require go 1.18 or later" when I try to build code with type parameters. Do I need a build flag?

Kind of a weird error to see when go version reports:
go version go1.18rc1 linux/amd64

The troubleshooting in the #generics Slack channel went back and forth a bit on whether or not the right version of Go was being used. Someone else later suggested that the go.mod language version needed to be updated, which was the issue.

I poked briefly at this using tip.

Friendly error

You get a friendly message suggesting to "check go.mod" if you instantiate a generic type or func:

type instantiation requires go1.18 or later (-lang was set to go1.17; check go.mod)

Example: https://go.dev/play/p/aXHOlSclSgT?v=gotip

Less friendly error

However, if you only declare a generic type or func without instantiation, you just get the following error (without the friendly suggestion to check go.mod):

type parameters require go1.18 or later

Example: https://go.dev/play/p/2Q1ZHdBWiPG?v=gotip

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FWIW, it's clear many more gophers are going to be using generics soon 😅, and addressing this might be a small bit of polish that could avoid some toe stubbing (and perhaps head off some issues getting filed or questions on the mailing list and so on).

Obviously, someone will eventually instantiate something, but it is probably not unusual to start with a declaration first and try to compile before trying to instantiate.

[thepudds]

FWIW, I have a cmd/compile CL that seems to address the specific examples above, which I will hopefully send in a bit.

[gopherbot]

Change https://go.dev/cl/390578 mentions this issue: cmd/compile/internal/types2: more consistently provide hint to check go.mod if language version < 1.18

[thepudds]

Hi @dmitshur and @findleyr, I don’t have a precise sense of (a) how close release is and (b) how high the bar is for changes for 1.18…

I suspect the answers are (a) “very close“ and (b) “very high“, but I at least wanted to ask if this issue is worth at least evaluating briefly.

FWIW, the number of gophers using generics is probably about to increase by a large factor, with a diversity of skillsets and experience (vs. the early adopters playing with them so far on average might tend to be more experienced).

Regarding the new message, one of the gophers in the #generics Slack channel commented:

Such a small thing, but this is likely to be a huge UX improvement for folks coming to 1.18 without full context.

In any event, the answer probably is “not even close“, but at least wanted to ask.

[findleyr]

CC @griesemer

I am not sure what meets the bar at this point, as we have been focusing on release blockers. In my opinion this change could be safe to cherry-pick, but I defer to the release team.

[dmitshur]

@thepudds Thanks for flagging this issue fix as potentially worth retargeting for Go 1.18. (We are close to the final release, and the bar is very high as described here.)

Having looked at the issue and CL closely: it's fixing an error message, and it will be most useful during people's initial experience with generics. The fix looks safe to me, has already gone through code review. If it's okay with @findleyr and @griesemer, I believe there's room to fit it in, so I'll tentatively move this to 1.18 milestone and insert it in #51460. (Our other options will be to consider it for backport to a 1.18.1 or later minor release, or leave it for 1.19.)