[dev.typeparams] go/types: export the Config.GoVersion field

Export the types.Config.GoVersion field, so that users can specify a
language compatibility version for go/types to enforce.

Updates #46648

Change-Id: I9e00122925faf0006cfb08c3f2d022619d5d54d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/334533
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/api.go

@@ -103,12 +103,12 @@ type ImporterFrom interface {
 // A Config specifies the configuration for type checking.
 // The zero value for Config is a ready-to-use default configuration.
 type Config struct {
-	// goVersion describes the accepted Go language version. The string
+	// GoVersion describes the accepted Go language version. The string
 	// must follow the format "go%d.%d" (e.g. "go1.12") or it must be
 	// empty; an empty string indicates the latest language version.
 	// If the format is invalid, invoking the type checker will cause a
 	// panic.
-	goVersion string
+	GoVersion string
 
 	// If IgnoreFuncBodies is set, function bodies are not
 	// type-checked.

src/go/types/check.go

@@ -180,9 +180,9 @@ func NewChecker(conf *Config, fset *token.FileSet, pkg *Package, info *Info) *Ch
 		info = new(Info)
 	}
 
-	version, err := parseGoVersion(conf.goVersion)
+	version, err := parseGoVersion(conf.GoVersion)
 	if err != nil {
-		panic(fmt.Sprintf("invalid Go version %q (%v)", conf.goVersion, err))
+		panic(fmt.Sprintf("invalid Go version %q (%v)", conf.GoVersion, err))
 	}
 
 	return &Checker{

src/go/types/check_test.go

@@ -244,7 +244,7 @@ func testFiles(t *testing.T, sizes Sizes, filenames []string, srcs [][]byte, man
 	// typecheck and collect typechecker errors
 	var conf Config
 	conf.Sizes = sizes
-	SetGoVersion(&conf, goVersion)
+	conf.GoVersion = goVersion
 
 	// special case for importC.src
 	if len(filenames) == 1 {

src/go/types/stdlib_test.go

@@ -140,8 +140,7 @@ func testTestDir(t *testing.T, path string, ignore ...string) {
 		// parse and type-check file
 		file, err := parser.ParseFile(fset, filename, nil, 0)
 		if err == nil {
-			conf := Config{Importer: stdLibImporter}
-			SetGoVersion(&conf, goVersion)
+			conf := Config{GoVersion: goVersion, Importer: stdLibImporter}
 			_, err = conf.Check(filename, fset, []*ast.File{file}, nil)
 		}
 

src/go/types/types_test.go

@@ -4,11 +4,5 @@
 
 package types
 
-// SetGoVersion sets the unexported goVersion field on config, so that tests
-// which assert on behavior for older Go versions can set it.
-func SetGoVersion(config *Config, goVersion string) {
-	config.goVersion = goVersion
-}
-
 // Debug is set if go/types is built with debug mode enabled.
 const Debug = debug