cmd/compile: Go 1.13 language changes not rejected in go1.12 mode

[rsc]

go tool compile -lang=go1.12 should probably be diagnosing (rejecting) use of the new Go 1.13 language features like signed shifts and the new number literals. That way people who have go 1.12 in their go.mod files can be more confident that the code does build with Go 1.12.

/cc @ianlancetaylor @griesemer

[dmitshur]

Would that apply in all .go files, even ones with // +build go1.13 build constraints?

An implication of doing this would be that, for example the go-cmp module that currently has a go 1.8 directive in its go.mod file, would not be able to use new Go 1.13 language features, even in .go files with a // +build go1.13 build constraint. /cc @dsnet

Is that an intended consequence? Is it unavoidable for modules that want to support more than 1 version of Go, or is go-cmp using a suboptimal value for the go directive (i.e., perhaps it should start using 1.12 or 1.13 once that's out).

[rsc]

This is independent of build tags. It's a little confusing but build tags are really about the specific release you are actually compiling with (usually for things like library features), while the go line is about what version of Go-the-language your source code intends to use. (It would be nice to do more for the library but that's future work at best.)

Discussed with @griesemer and @ianlancetaylor; Robert will check on it. Accepted.

[gopherbot]

Change https://golang.org/cl/174897 mentions this issue: cmd/compile: disable Go1.13 language features for -lang=go1.12 and below

[gopherbot]

Change https://golang.org/cl/198491 mentions this issue: cmd/compile: better error message for language version errors