cmd/go: `go mod init' from 1.21 incompatible with older versions

[howardjohn]

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

$ go version
go version go1.21.0 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

Using 1.21's go mod init:

$ cd `mktemp -d`
$ go1.21.0 mod init test
$ go1.21.0 build
no Go files in ...
$ go1.20 build
go: errors parsing go.mod:
/tmp/tmp.eoz8RDYcBC/go.mod:3: invalid go version '1.21.0': must match format 1.23

We can see the result is not compatible with older versions

If we manually set the go.mod up with the same semantics, it works fine:

$ cd `mktemp -d`
$ cat <<EOF > go.mod
module test

go 1.21
EOF
$ go1.21.0 build
no Go files in ...
$ go1.20 build
no Go files in ...

What did you expect to see?

go mod init created by 1.21 can be built by older Go versions

What did you see instead?

Somewhat obscure error

[bcmills]

In general a go.mod file (or Go source file) created for a higher Go version cannot be expected to successfully build with an older go version.

Starting with Go 1.21, you can use go get [email protected] or similar to explicitly downgrade for compatibility with an older release.

(You can also use the older go mod tidy flag for that, like go mod tidy -go=1.20, which works with older Go releases back to 1.17, although go get is preferred going forward.)