package loader loads `./...` differently than Go tools' loader?

[garysferrao]

background

i came across an interesting case in this thread on Go's slack channel: https://gophers.slack.com/archives/C029RQSEE/p1755713320936029

GitLab CI would like us to use GOPATH: $CI_PROJECT_DIR/.go if we'd like to use Go modules cache: https://docs.gitlab.com/ci/caching/#cache-go-dependencies

but this causes an error when trying to list ./...:

package main

import (
	"fmt"
	"os"
	"path/filepath"
	"strings"

	"golang.org/x/tools/go/packages"
	"sigs.k8s.io/controller-tools/pkg/loader"
)

func main() {
	// allow environment (GOMODCACHE/GOPATH) to influence loading
	cfg := &packages.Config{
		Mode: packages.NeedName | packages.NeedFiles | packages.NeedModule | packages.NeedDeps,
		Env:  os.Environ(),
		Dir:  ".",
	}

	pkgs, err := packages.Load(cfg, "./...")
	if err != nil {
		fmt.Fprintf(os.Stderr, "packages.Load error: %v\n", err)
		os.Exit(2)
	}
	fmt.Printf("discovered %d root packages\n\n%", len(pkgs))
	for _, p := range pkgs {
		modDir := "<none>"
		modPath := "<none>"
		if p.Module != nil {
			modDir = p.Module.Dir
			modPath = p.Module.Path
		}
		fmt.Printf("pkg: %-40s module.path: %-40s module.dir: %s\n",
			p.PkgPath, modPath, modDir)

		// quick detection for module-cache location
		if modDir != "<none>" && strings.Contains(modDir, string(filepath.Separator)+"pkg"+string(filepath.Separator)+"mod"+string(filepath.Separator)) {
			fmt.Printf("  -> note: this package's module dir looks like it came from pkg/mod (module cache)\n")
		}
		if len(p.Errors) > 0 {
			fmt.Printf("  -> package had %d error(s)\n", len(p.Errors))
			for _, e := range p.Errors {
				fmt.Printf("     - %v\n", e)
			}
		}
	}

	fmt.Println("\n\n")

	// load roots just like controller-gen would do for "./..."
	pkgs2, err := loader.LoadRootsWithConfig(cfg, "./...")
	if err != nil {
		fmt.Fprintf(os.Stderr, "loader.LoadRootsWithConfig error: %v\n", err)
		os.Exit(2)
	}

	fmt.Printf("discovered %d root packages\n\n", len(pkgs2))
	for _, p := range pkgs2 {
		modDir := "<none>"
		modPath := "<none>"
		if p.Module != nil {
			modDir = p.Module.Dir
			modPath = p.Module.Path
		}
		fmt.Printf("pkg: %-40s module.path: %-40s module.dir: %s\n",
			p.PkgPath, modPath, modDir)

		// quick detection for module-cache location
		if modDir != "<none>" && strings.Contains(modDir, string(filepath.Separator)+"pkg"+string(filepath.Separator)+"mod"+string(filepath.Separator)) {
			fmt.Printf("  -> note: this package's module dir looks like it came from pkg/mod (module cache)\n")
		}
		if len(p.Errors) > 0 {
			fmt.Printf("  -> package had %d error(s)\n", len(p.Errors))
			for _, e := range p.Errors {
				fmt.Printf("     - %v\n", e)
			}
		}
	}
}

loader.LoadRootsWithConfig error: load packages in root ".go/gopkg.in/yaml.v2@v2.4.0": err: exit status 1: stderr: go: gopkg.in/check.v1@v0.0.0-20161208181325-20d25e280405: missing go.sum entry for go.mod file; to add it:
        go mod download gopkg.in/check.v1

whereas the golang.org/x/tools/go/packages's Load() can print the list much faster and without errors.

---

would this be a known behaviour or is it a bug?