What version of CUE are you using (cue version)?

$ cue version
v0.9.0

Does this issue reproduce with the latest release?

Yes

What did you do?

Consider the following, where the CUE module in the m directory is the result of downloading a module that has previously been published somewhere (e.g. an OCI registry). Note the module is missing a language.version: either because it was published prior to v0.8.0 (which introduced that field via v0.8.0-alpha.3) or because a tool using the Go API did not add such a line. The Go code below is in effect a simulation of a tool like Timoni (https://timoni.sh/) that uses the Go API, and consumes CUE modules that have been published someone, but not published via the mechanism that became the default in v0.9.0.

go mod tidy
go run .
cmp stdout stdout.golden

-- go.mod --
module mod.example

go 1.22.3

require cuelang.org/go v0.9.0

-- m/cue.mod/module.cue --
module: "mod.example/blah"

-- m/blah.cue --
package blah

blah: 42

-- main.go --
package main

import (
	"fmt"
	"log"

	"cuelang.org/go/cue/cuecontext"
	"cuelang.org/go/cue/load"
)

func main() {
	ctx := cuecontext.New()
	bis := load.Instances([]string{"."}, &load.Config{
		Dir: "./m",
	})
	v := ctx.BuildInstance(bis[0])
	if err := v.Err(); err != nil {
		log.Fatal(err)
	}
	fmt.Println(v)
}
-- stdout.golden --
{
	blah: 42
}

What did you expect to see?

The test to pass.

What did you see instead?

> go mod tidy
> go run .
[stderr]
main.go:18: no language version declared in module.cue
exit status 1

[exit status 1]
FAIL: /tmp/testscript3849746612/repro.txtar/script.txtar:2: unexpected go command failure

When we were considering the enforcing of language.version, we did not consider the case of pre-existing modules that have been published somewhere without a language.version. Instead our analysis focussed on consideration of those who had been using the modules experiment that started with the v0.8.0 alphas. This was a mistake. Because this Go code not working results in a breaking change for pre-existing modules of CUE that are published (which one should read as "and would need to be republished to add a language.version). This affects tools written against the Go API which themselves provide a means of downloading a published CUE module.

We are therefore proposing a fix via a flag for cue/load.Config that, when enabled, will enabled legacy modules without a language.version to be loaded.