cmd/go: "go mod tidy" is not the only subcommand to add "go 1.12" to go.mod

[neild]

The 1.12 release notes say:

The go directive in a go.mod file now indicates the version of the language used by the files within that module, and go mod tidy sets it to the current release (go 1.12) if no existing version is present.

The version is not just being added by go mod tidy, but also go build and go test.

$ echo "module github.com/neild/foo" > go.mod
$ echo "package foo" > foo.go
$ go test .
?       github.com/neild/foo    [no test files]
$ cat go.mod
module github.com/neild/foo

go 1.12

I don't know if the relnotes or the behavior is in error.

[mvdan]

/cc @bcmills @ianlancetaylor

[gopherbot]

Change https://golang.org/cl/160797 mentions this issue: doc: go1.12: update notes on go directive

[thepudds]

I know this is closed, but FWIW, it is not obvious to me what triggers adding a missing go version directive. go mod init adds it. Things like go build and go mod tidy do not seem to add a missing go version directive, with seemingly the exception if you do not have any require statements then go build or go mod tidy seem to add a missing go version directive.

The draft 1.12 release notes used to say go mod tidy is what would trigger adding a missing go version directive, but that was seemingly removed in the final version of the release notes:

https://go-review.googlesource.com/c/go/+/160797/2/doc/go1.12.html

CC @gilcrest