go/loader: resolve imports in "created" packages w.r.t. parent dir not cwd

Fixes golang/go#16580

Change-Id: Id891f40659257eac9dbb0d70dae17f725e911f9b
Reviewed-on: https://go-review.googlesource.com/33589
Reviewed-by: Robert Griesemer <gri@golang.org>

go/loader/loader.go

@@ -17,6 +17,7 @@ import (
 	"go/token"
 	"go/types"
 	"os"
+	"path/filepath"
 	"sort"
 	"strings"
 	"sync"
@@ -132,6 +133,8 @@ type Config struct {
 // Files are processed first, but typically only one of Files and
 // Filenames is provided.  The path needn't be globally unique.
 //
+// For vendoring purposes, the package's directory is the one that
+// contains the first file.
 type PkgSpec struct {
 	Path      string      // package path ("" => use package declaration)
 	Files     []*ast.File // ASTs of already-parsed files
@@ -586,9 +589,8 @@ func (conf *Config) Load() (*Program, error) {
 		imp.addFiles(info, files, false)
 	}
 
-	createPkg := func(path string, files []*ast.File, errs []error) {
-		// TODO(adonovan): fix: use dirname of files, not cwd.
-		info := imp.newPackageInfo(path, conf.Cwd)
+	createPkg := func(path, dir string, files []*ast.File, errs []error) {
+		info := imp.newPackageInfo(path, dir)
 		for _, err := range errs {
 			info.appendError(err)
 		}
@@ -613,14 +615,19 @@ func (conf *Config) Load() (*Program, error) {
 				path = "(unnamed)"
 			}
 		}
-		createPkg(path, files, errs)
+
+		dir := "."
+		if len(files) > 0 && files[0].Pos().IsValid() {
+			dir = filepath.Dir(conf.fset().File(files[0].Pos()).Name())
+		}
+		createPkg(path, dir, files, errs)
 	}
 
 	// Create external test packages.
 	sort.Sort(byImportPath(xtestPkgs))
 	for _, bp := range xtestPkgs {
 		files, errs := imp.conf.parsePackageFiles(bp, 'x')
-		createPkg(bp.ImportPath+"_test", files, errs)
+		createPkg(bp.ImportPath+"_test", bp.Dir, files, errs)
 	}
 
 	// -- finishing up (sequential) ----------------------------------------

go/loader/loader_test.go

@@ -13,6 +13,8 @@ package loader_test
 import (
 	"fmt"
 	"go/build"
+	"go/constant"
+	"go/types"
 	"path/filepath"
 	"reflect"
 	"sort"
@@ -477,6 +479,60 @@ func TestVendorCwd(t *testing.T) {
 	}
 }
 
+func TestVendorCwdIssue16580(t *testing.T) {
+	// Regression test for Go issue 16580.
+	// Import decls in "created" packages were vendor-resolved
+	// w.r.t. cwd, not the parent directory of the package's files.
+	ctxt := fakeContext(map[string]string{
+		"a":          ``, // mkdir a
+		"a/vendor":   ``, // mkdir a/vendor
+		"a/vendor/b": `package b; const X = true`,
+		"b":          `package b; const X = false`,
+	})
+	for _, test := range []struct {
+		filename, cwd string
+		want          bool // expected value of b.X; depends on filename, not on cwd
+	}{
+		{filename: "c.go", cwd: "/go/src", want: false},
+		{filename: "c.go", cwd: "/go/src/a", want: false},
+		{filename: "c.go", cwd: "/go/src/a/b", want: false},
+		{filename: "c.go", cwd: "/go/src/a/vendor/b", want: false},
+
+		{filename: "/go/src/a/c.go", cwd: "/go/src", want: true},
+		{filename: "/go/src/a/c.go", cwd: "/go/src/a", want: true},
+		{filename: "/go/src/a/c.go", cwd: "/go/src/a/b", want: true},
+		{filename: "/go/src/a/c.go", cwd: "/go/src/a/vendor/b", want: true},
+
+		{filename: "/go/src/c/c.go", cwd: "/go/src", want: false},
+		{filename: "/go/src/c/c.go", cwd: "/go/src/a", want: false},
+		{filename: "/go/src/c/c.go", cwd: "/go/src/a/b", want: false},
+		{filename: "/go/src/c/c.go", cwd: "/go/src/a/vendor/b", want: false},
+	} {
+		conf := loader.Config{
+			Cwd:   test.cwd,
+			Build: ctxt,
+		}
+		f, err := conf.ParseFile(test.filename, `package dummy; import "b"; const X = b.X`)
+		if err != nil {
+			t.Fatal(f)
+		}
+		conf.CreateFromFiles("dummy", f)
+
+		prog, err := conf.Load()
+		if err != nil {
+			t.Errorf("%+v: Load failed: %v", test, err)
+			continue
+		}
+
+		x := constant.BoolVal(prog.Created[0].Pkg.Scope().Lookup("X").(*types.Const).Val())
+		if x != test.want {
+			t.Errorf("%+v: b.X = %t", test, x)
+		}
+	}
+
+	// TODO(adonovan): also test imports within XTestGoFiles.
+}
+
 // TODO(adonovan): more Load tests:
 //
 // failures: