diff --git a/cmd/stringer/stringer.go b/cmd/stringer/stringer.go
index 4afecd6f..992125ab 100644
--- a/cmd/stringer/stringer.go
+++ b/cmd/stringer/stringer.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.5
+
 // Stringer is a tool to automate the creation of methods that satisfy the fmt.Stringer
 // interface. Given the name of a (signed or unsigned) integer type T that has constants
 // defined, stringer will create a new self-contained Go source file implementing
diff --git a/cmd/stringer/stringer14.go b/cmd/stringer/stringer14.go
new file mode 100644
index 00000000..e0962e3f
--- /dev/null
+++ b/cmd/stringer/stringer14.go
@@ -0,0 +1,640 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !go1.5
+
+// Stringer is a tool to automate the creation of methods that satisfy the fmt.Stringer
+// interface. Given the name of a (signed or unsigned) integer type T that has constants
+// defined, stringer will create a new self-contained Go source file implementing
+//	func (t T) String() string
+// The file is created in the same package and directory as the package that defines T.
+// It has helpful defaults designed for use with go generate.
+//
+// Stringer works best with constants that are consecutive values such as created using iota,
+// but creates good code regardless. In the future it might also provide custom support for
+// constant sets that are bit patterns.
+//
+// For example, given this snippet,
+//
+//	package painkiller
+//
+//	type Pill int
+//
+//	const (
+//		Placebo Pill = iota
+//		Aspirin
+//		Ibuprofen
+//		Paracetamol
+//		Acetaminophen = Paracetamol
+//	)
+//
+// running this command
+//
+//	stringer -type=Pill
+//
+// in the same directory will create the file pill_string.go, in package painkiller,
+// containing a definition of
+//
+//	func (Pill) String() string
+//
+// That method will translate the value of a Pill constant to the string representation
+// of the respective constant name, so that the call fmt.Print(painkiller.Aspirin) will
+// print the string "Aspirin".
+//
+// Typically this process would be run using go generate, like this:
+//
+//	//go:generate stringer -type=Pill
+//
+// If multiple constants have the same value, the lexically first matching name will
+// be used (in the example, Acetaminophen will print as "Paracetamol").
+//
+// With no arguments, it processes the package in the current directory.
+// Otherwise, the arguments must name a single directory holding a Go package
+// or a set of Go source files that represent a single Go package.
+//
+// The -type flag accepts a comma-separated list of types so a single run can
+// generate methods for multiple types. The default output file is t_string.go,
+// where t is the lower-cased name of the first type listed. It can be overridden
+// with the -output flag.
+//
+package main // import "golang.org/x/tools/cmd/stringer"
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"golang.org/x/tools/go/exact"
+	"golang.org/x/tools/go/types"
+
+	_ "golang.org/x/tools/go/gcimporter"
+)
+
+var (
+	typeNames = flag.String("type", "", "comma-separated list of type names; must be set")
+	output    = flag.String("output", "", "output file name; default srcdir/<type>_string.go")
+)
+
+// Usage is a replacement usage function for the flags package.
+func Usage() {
+	fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
+	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T [directory]\n")
+	fmt.Fprintf(os.Stderr, "\tstringer [flags[ -type T files... # Must be a single package\n")
+	fmt.Fprintf(os.Stderr, "For more information, see:\n")
+	fmt.Fprintf(os.Stderr, "\thttp://godoc.org/golang.org/x/tools/cmd/stringer\n")
+	fmt.Fprintf(os.Stderr, "Flags:\n")
+	flag.PrintDefaults()
+}
+
+func main() {
+	log.SetFlags(0)
+	log.SetPrefix("stringer: ")
+	flag.Usage = Usage
+	flag.Parse()
+	if len(*typeNames) == 0 {
+		flag.Usage()
+		os.Exit(2)
+	}
+	types := strings.Split(*typeNames, ",")
+
+	// We accept either one directory or a list of files. Which do we have?
+	args := flag.Args()
+	if len(args) == 0 {
+		// Default: process whole package in current directory.
+		args = []string{"."}
+	}
+
+	// Parse the package once.
+	var (
+		dir string
+		g   Generator
+	)
+	if len(args) == 1 && isDirectory(args[0]) {
+		dir = args[0]
+		g.parsePackageDir(args[0])
+	} else {
+		dir = filepath.Dir(args[0])
+		g.parsePackageFiles(args)
+	}
+
+	// Print the header and package clause.
+	g.Printf("// Code generated by \"stringer %s\"; DO NOT EDIT\n", strings.Join(os.Args[1:], " "))
+	g.Printf("\n")
+	g.Printf("package %s", g.pkg.name)
+	g.Printf("\n")
+	g.Printf("import \"fmt\"\n") // Used by all methods.
+
+	// Run generate for each type.
+	for _, typeName := range types {
+		g.generate(typeName)
+	}
+
+	// Format the output.
+	src := g.format()
+
+	// Write to file.
+	outputName := *output
+	if outputName == "" {
+		baseName := fmt.Sprintf("%s_string.go", types[0])
+		outputName = filepath.Join(dir, strings.ToLower(baseName))
+	}
+	err := ioutil.WriteFile(outputName, src, 0644)
+	if err != nil {
+		log.Fatalf("writing output: %s", err)
+	}
+}
+
+// isDirectory reports whether the named file is a directory.
+func isDirectory(name string) bool {
+	info, err := os.Stat(name)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return info.IsDir()
+}
+
+// Generator holds the state of the analysis. Primarily used to buffer
+// the output for format.Source.
+type Generator struct {
+	buf bytes.Buffer // Accumulated output.
+	pkg *Package     // Package we are scanning.
+}
+
+func (g *Generator) Printf(format string, args ...interface{}) {
+	fmt.Fprintf(&g.buf, format, args...)
+}
+
+// File holds a single parsed file and associated data.
+type File struct {
+	pkg  *Package  // Package to which this file belongs.
+	file *ast.File // Parsed AST.
+	// These fields are reset for each type being generated.
+	typeName string  // Name of the constant type.
+	values   []Value // Accumulator for constant values of that type.
+}
+
+type Package struct {
+	dir      string
+	name     string
+	defs     map[*ast.Ident]types.Object
+	files    []*File
+	typesPkg *types.Package
+}
+
+// parsePackageDir parses the package residing in the directory.
+func (g *Generator) parsePackageDir(directory string) {
+	pkg, err := build.Default.ImportDir(directory, 0)
+	if err != nil {
+		log.Fatalf("cannot process directory %s: %s", directory, err)
+	}
+	var names []string
+	names = append(names, pkg.GoFiles...)
+	names = append(names, pkg.CgoFiles...)
+	// TODO: Need to think about constants in test files. Maybe write type_string_test.go
+	// in a separate pass? For later.
+	// names = append(names, pkg.TestGoFiles...) // These are also in the "foo" package.
+	names = append(names, pkg.SFiles...)
+	names = prefixDirectory(directory, names)
+	g.parsePackage(directory, names, nil)
+}
+
+// parsePackageFiles parses the package occupying the named files.
+func (g *Generator) parsePackageFiles(names []string) {
+	g.parsePackage(".", names, nil)
+}
+
+// prefixDirectory places the directory name on the beginning of each name in the list.
+func prefixDirectory(directory string, names []string) []string {
+	if directory == "." {
+		return names
+	}
+	ret := make([]string, len(names))
+	for i, name := range names {
+		ret[i] = filepath.Join(directory, name)
+	}
+	return ret
+}
+
+// parsePackage analyzes the single package constructed from the named files.
+// If text is non-nil, it is a string to be used instead of the content of the file,
+// to be used for testing. parsePackage exits if there is an error.
+func (g *Generator) parsePackage(directory string, names []string, text interface{}) {
+	var files []*File
+	var astFiles []*ast.File
+	g.pkg = new(Package)
+	fs := token.NewFileSet()
+	for _, name := range names {
+		if !strings.HasSuffix(name, ".go") {
+			continue
+		}
+		parsedFile, err := parser.ParseFile(fs, name, text, 0)
+		if err != nil {
+			log.Fatalf("parsing package: %s: %s", name, err)
+		}
+		astFiles = append(astFiles, parsedFile)
+		files = append(files, &File{
+			file: parsedFile,
+			pkg:  g.pkg,
+		})
+	}
+	if len(astFiles) == 0 {
+		log.Fatalf("%s: no buildable Go files", directory)
+	}
+	g.pkg.name = astFiles[0].Name.Name
+	g.pkg.files = files
+	g.pkg.dir = directory
+	// Type check the package.
+	g.pkg.check(fs, astFiles)
+}
+
+// check type-checks the package. The package must be OK to proceed.
+func (pkg *Package) check(fs *token.FileSet, astFiles []*ast.File) {
+	pkg.defs = make(map[*ast.Ident]types.Object)
+	config := types.Config{FakeImportC: true}
+	info := &types.Info{
+		Defs: pkg.defs,
+	}
+	typesPkg, err := config.Check(pkg.dir, fs, astFiles, info)
+	if err != nil {
+		log.Fatalf("checking package: %s", err)
+	}
+	pkg.typesPkg = typesPkg
+}
+
+// generate produces the String method for the named type.
+func (g *Generator) generate(typeName string) {
+	values := make([]Value, 0, 100)
+	for _, file := range g.pkg.files {
+		// Set the state for this run of the walker.
+		file.typeName = typeName
+		file.values = nil
+		if file.file != nil {
+			ast.Inspect(file.file, file.genDecl)
+			values = append(values, file.values...)
+		}
+	}
+
+	if len(values) == 0 {
+		log.Fatalf("no values defined for type %s", typeName)
+	}
+	runs := splitIntoRuns(values)
+	// The decision of which pattern to use depends on the number of
+	// runs in the numbers. If there's only one, it's easy. For more than
+	// one, there's a tradeoff between complexity and size of the data
+	// and code vs. the simplicity of a map. A map takes more space,
+	// but so does the code. The decision here (crossover at 10) is
+	// arbitrary, but considers that for large numbers of runs the cost
+	// of the linear scan in the switch might become important, and
+	// rather than use yet another algorithm such as binary search,
+	// we punt and use a map. In any case, the likelihood of a map
+	// being necessary for any realistic example other than bitmasks
+	// is very low. And bitmasks probably deserve their own analysis,
+	// to be done some other day.
+	switch {
+	case len(runs) == 1:
+		g.buildOneRun(runs, typeName)
+	case len(runs) <= 10:
+		g.buildMultipleRuns(runs, typeName)
+	default:
+		g.buildMap(runs, typeName)
+	}
+}
+
+// splitIntoRuns breaks the values into runs of contiguous sequences.
+// For example, given 1,2,3,5,6,7 it returns {1,2,3},{5,6,7}.
+// The input slice is known to be non-empty.
+func splitIntoRuns(values []Value) [][]Value {
+	// We use stable sort so the lexically first name is chosen for equal elements.
+	sort.Stable(byValue(values))
+	// Remove duplicates. Stable sort has put the one we want to print first,
+	// so use that one. The String method won't care about which named constant
+	// was the argument, so the first name for the given value is the only one to keep.
+	// We need to do this because identical values would cause the switch or map
+	// to fail to compile.
+	j := 1
+	for i := 1; i < len(values); i++ {
+		if values[i].value != values[i-1].value {
+			values[j] = values[i]
+			j++
+		}
+	}
+	values = values[:j]
+	runs := make([][]Value, 0, 10)
+	for len(values) > 0 {
+		// One contiguous sequence per outer loop.
+		i := 1
+		for i < len(values) && values[i].value == values[i-1].value+1 {
+			i++
+		}
+		runs = append(runs, values[:i])
+		values = values[i:]
+	}
+	return runs
+}
+
+// format returns the gofmt-ed contents of the Generator's buffer.
+func (g *Generator) format() []byte {
+	src, err := format.Source(g.buf.Bytes())
+	if err != nil {
+		// Should never happen, but can arise when developing this code.
+		// The user can compile the output to see the error.
+		log.Printf("warning: internal error: invalid Go generated: %s", err)
+		log.Printf("warning: compile the package to analyze the error")
+		return g.buf.Bytes()
+	}
+	return src
+}
+
+// Value represents a declared constant.
+type Value struct {
+	name string // The name of the constant.
+	// The value is stored as a bit pattern alone. The boolean tells us
+	// whether to interpret it as an int64 or a uint64; the only place
+	// this matters is when sorting.
+	// Much of the time the str field is all we need; it is printed
+	// by Value.String.
+	value  uint64 // Will be converted to int64 when needed.
+	signed bool   // Whether the constant is a signed type.
+	str    string // The string representation given by the "go/exact" package.
+}
+
+func (v *Value) String() string {
+	return v.str
+}
+
+// byValue lets us sort the constants into increasing order.
+// We take care in the Less method to sort in signed or unsigned order,
+// as appropriate.
+type byValue []Value
+
+func (b byValue) Len() int      { return len(b) }
+func (b byValue) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
+func (b byValue) Less(i, j int) bool {
+	if b[i].signed {
+		return int64(b[i].value) < int64(b[j].value)
+	}
+	return b[i].value < b[j].value
+}
+
+// genDecl processes one declaration clause.
+func (f *File) genDecl(node ast.Node) bool {
+	decl, ok := node.(*ast.GenDecl)
+	if !ok || decl.Tok != token.CONST {
+		// We only care about const declarations.
+		return true
+	}
+	// The name of the type of the constants we are declaring.
+	// Can change if this is a multi-element declaration.
+	typ := ""
+	// Loop over the elements of the declaration. Each element is a ValueSpec:
+	// a list of names possibly followed by a type, possibly followed by values.
+	// If the type and value are both missing, we carry down the type (and value,
+	// but the "go/types" package takes care of that).
+	for _, spec := range decl.Specs {
+		vspec := spec.(*ast.ValueSpec) // Guaranteed to succeed as this is CONST.
+		if vspec.Type == nil && len(vspec.Values) > 0 {
+			// "X = 1". With no type but a value, the constant is untyped.
+			// Skip this vspec and reset the remembered type.
+			typ = ""
+			continue
+		}
+		if vspec.Type != nil {
+			// "X T". We have a type. Remember it.
+			ident, ok := vspec.Type.(*ast.Ident)
+			if !ok {
+				continue
+			}
+			typ = ident.Name
+		}
+		if typ != f.typeName {
+			// This is not the type we're looking for.
+			continue
+		}
+		// We now have a list of names (from one line of source code) all being
+		// declared with the desired type.
+		// Grab their names and actual values and store them in f.values.
+		for _, name := range vspec.Names {
+			if name.Name == "_" {
+				continue
+			}
+			// This dance lets the type checker find the values for us. It's a
+			// bit tricky: look up the object declared by the name, find its
+			// types.Const, and extract its value.
+			obj, ok := f.pkg.defs[name]
+			if !ok {
+				log.Fatalf("no value for constant %s", name)
+			}
+			info := obj.Type().Underlying().(*types.Basic).Info()
+			if info&types.IsInteger == 0 {
+				log.Fatalf("can't handle non-integer constant type %s", typ)
+			}
+			value := obj.(*types.Const).Val() // Guaranteed to succeed as this is CONST.
+			if value.Kind() != exact.Int {
+				log.Fatalf("can't happen: constant is not an integer %s", name)
+			}
+			i64, isInt := exact.Int64Val(value)
+			u64, isUint := exact.Uint64Val(value)
+			if !isInt && !isUint {
+				log.Fatalf("internal error: value of %s is not an integer: %s", name, value.String())
+			}
+			if !isInt {
+				u64 = uint64(i64)
+			}
+			v := Value{
+				name:   name.Name,
+				value:  u64,
+				signed: info&types.IsUnsigned == 0,
+				str:    value.String(),
+			}
+			f.values = append(f.values, v)
+		}
+	}
+	return false
+}
+
+// Helpers
+
+// usize returns the number of bits of the smallest unsigned integer
+// type that will hold n. Used to create the smallest possible slice of
+// integers to use as indexes into the concatenated strings.
+func usize(n int) int {
+	switch {
+	case n < 1<<8:
+		return 8
+	case n < 1<<16:
+		return 16
+	default:
+		// 2^32 is enough constants for anyone.
+		return 32
+	}
+}
+
+// declareIndexAndNameVars declares the index slices and concatenated names
+// strings representing the runs of values.
+func (g *Generator) declareIndexAndNameVars(runs [][]Value, typeName string) {
+	var indexes, names []string
+	for i, run := range runs {
+		index, name := g.createIndexAndNameDecl(run, typeName, fmt.Sprintf("_%d", i))
+		indexes = append(indexes, index)
+		names = append(names, name)
+	}
+	g.Printf("const (\n")
+	for _, name := range names {
+		g.Printf("\t%s\n", name)
+	}
+	g.Printf(")\n\n")
+	g.Printf("var (")
+	for _, index := range indexes {
+		g.Printf("\t%s\n", index)
+	}
+	g.Printf(")\n\n")
+}
+
+// declareIndexAndNameVar is the single-run version of declareIndexAndNameVars
+func (g *Generator) declareIndexAndNameVar(run []Value, typeName string) {
+	index, name := g.createIndexAndNameDecl(run, typeName, "")
+	g.Printf("const %s\n", name)
+	g.Printf("var %s\n", index)
+}
+
+// createIndexAndNameDecl returns the pair of declarations for the run. The caller will add "const" and "var".
+func (g *Generator) createIndexAndNameDecl(run []Value, typeName string, suffix string) (string, string) {
+	b := new(bytes.Buffer)
+	indexes := make([]int, len(run))
+	for i := range run {
+		b.WriteString(run[i].name)
+		indexes[i] = b.Len()
+	}
+	nameConst := fmt.Sprintf("_%s_name%s = %q", typeName, suffix, b.String())
+	nameLen := b.Len()
+	b.Reset()
+	fmt.Fprintf(b, "_%s_index%s = [...]uint%d{0, ", typeName, suffix, usize(nameLen))
+	for i, v := range indexes {
+		if i > 0 {
+			fmt.Fprintf(b, ", ")
+		}
+		fmt.Fprintf(b, "%d", v)
+	}
+	fmt.Fprintf(b, "}")
+	return b.String(), nameConst
+}
+
+// declareNameVars declares the concatenated names string representing all the values in the runs.
+func (g *Generator) declareNameVars(runs [][]Value, typeName string, suffix string) {
+	g.Printf("const _%s_name%s = \"", typeName, suffix)
+	for _, run := range runs {
+		for i := range run {
+			g.Printf("%s", run[i].name)
+		}
+	}
+	g.Printf("\"\n")
+}
+
+// buildOneRun generates the variables and String method for a single run of contiguous values.
+func (g *Generator) buildOneRun(runs [][]Value, typeName string) {
+	values := runs[0]
+	g.Printf("\n")
+	g.declareIndexAndNameVar(values, typeName)
+	// The generated code is simple enough to write as a Printf format.
+	lessThanZero := ""
+	if values[0].signed {
+		lessThanZero = "i < 0 || "
+	}
+	if values[0].value == 0 { // Signed or unsigned, 0 is still 0.
+		g.Printf(stringOneRun, typeName, usize(len(values)), lessThanZero)
+	} else {
+		g.Printf(stringOneRunWithOffset, typeName, values[0].String(), usize(len(values)), lessThanZero)
+	}
+}
+
+// Arguments to format are:
+//	[1]: type name
+//	[2]: size of index element (8 for uint8 etc.)
+//	[3]: less than zero check (for signed types)
+const stringOneRun = `func (i %[1]s) String() string {
+	if %[3]si >= %[1]s(len(_%[1]s_index)-1) {
+		return fmt.Sprintf("%[1]s(%%d)", i)
+	}
+	return _%[1]s_name[_%[1]s_index[i]:_%[1]s_index[i+1]]
+}
+`
+
+// Arguments to format are:
+//	[1]: type name
+//	[2]: lowest defined value for type, as a string
+//	[3]: size of index element (8 for uint8 etc.)
+//	[4]: less than zero check (for signed types)
+/*
+ */
+const stringOneRunWithOffset = `func (i %[1]s) String() string {
+	i -= %[2]s
+	if %[4]si >= %[1]s(len(_%[1]s_index)-1) {
+		return fmt.Sprintf("%[1]s(%%d)", i + %[2]s)
+	}
+	return _%[1]s_name[_%[1]s_index[i] : _%[1]s_index[i+1]]
+}
+`
+
+// buildMultipleRuns generates the variables and String method for multiple runs of contiguous values.
+// For this pattern, a single Printf format won't do.
+func (g *Generator) buildMultipleRuns(runs [][]Value, typeName string) {
+	g.Printf("\n")
+	g.declareIndexAndNameVars(runs, typeName)
+	g.Printf("func (i %s) String() string {\n", typeName)
+	g.Printf("\tswitch {\n")
+	for i, values := range runs {
+		if len(values) == 1 {
+			g.Printf("\tcase i == %s:\n", &values[0])
+			g.Printf("\t\treturn _%s_name_%d\n", typeName, i)
+			continue
+		}
+		g.Printf("\tcase %s <= i && i <= %s:\n", &values[0], &values[len(values)-1])
+		if values[0].value != 0 {
+			g.Printf("\t\ti -= %s\n", &values[0])
+		}
+		g.Printf("\t\treturn _%s_name_%d[_%s_index_%d[i]:_%s_index_%d[i+1]]\n",
+			typeName, i, typeName, i, typeName, i)
+	}
+	g.Printf("\tdefault:\n")
+	g.Printf("\t\treturn fmt.Sprintf(\"%s(%%d)\", i)\n", typeName)
+	g.Printf("\t}\n")
+	g.Printf("}\n")
+}
+
+// buildMap handles the case where the space is so sparse a map is a reasonable fallback.
+// It's a rare situation but has simple code.
+func (g *Generator) buildMap(runs [][]Value, typeName string) {
+	g.Printf("\n")
+	g.declareNameVars(runs, typeName, "")
+	g.Printf("\nvar _%s_map = map[%s]string{\n", typeName, typeName)
+	n := 0
+	for _, values := range runs {
+		for _, value := range values {
+			g.Printf("\t%s: _%s_name[%d:%d],\n", &value, typeName, n, n+len(value.name))
+			n += len(value.name)
+		}
+	}
+	g.Printf("}\n\n")
+	g.Printf(stringMap, typeName)
+}
+
+// Argument to format is the type name.
+const stringMap = `func (i %[1]s) String() string {
+	if str, ok := _%[1]s_map[i]; ok {
+		return str
+	}
+	return fmt.Sprintf("%[1]s(%%d)", i)
+}
+`