diff --git a/go/types/typemap/typemap.go b/go/types/typemap/typemap.go
new file mode 100644
index 00000000000..d8f1f109fd9
--- /dev/null
+++ b/go/types/typemap/typemap.go
@@ -0,0 +1,310 @@
+// Package typemap defines type M, a hash-table-based mapping from
+// types (go/types.Type) to arbitrary values, and a hash function on
+// types.
+//
+// The concrete types that implement the Type interface are pointers.
+// Since they are not canonicalized, == cannot be used to check for
+// equivalence, and thus we cannot simply use a Go map.
+//
+// Not thread-safe.
+//
+package typemap
+
+import (
+	"bytes"
+	"fmt"
+	"unsafe"
+
+	"code.google.com/p/go.tools/go/types"
+)
+
+// typemap.M is a mapping from types.Type to interface{} values.
+//
+// Just as with map[K]V, a nil *typemap.M is a valid empty map.
+//
+type M struct {
+	hasher Hasher             // shared by many typemap.Ms
+	table  map[uint32][]entry // maps hash to bucket; entry.key==nil means unused
+	length int                // number of map entries
+}
+
+// entry is an entry (key/value association) in a hash bucket.
+type entry struct {
+	key   types.Type
+	value interface{}
+}
+
+// SetHasher sets the hasher used by typemap.M.
+//
+// All Hashers are functionally equivalent but contain internal state
+// used to cache the results of hashing previously seen types.
+//
+// A single Hasher created by MakeHasher() may be shared among
+// many typemap.M instances.  This is recommended if the instances
+// have many keys in common, as it will amortize the cost of hash
+// computation.
+//
+// A Hasher may grow without bound as new types are seen.  Even when a
+// type is deleted from the map, the Hasher never shrinks, since other
+// types in the map may reference the deleted type indirectly.
+//
+// Hashers are not thread-safe, and read-only operations such as
+// M.Lookup require updates to the hasher, so a full Mutex lock (not a
+// read-lock) is require around all typemap.M operations if a shared
+// hasher is accessed from multiple threads.
+//
+// If SetHasher is not called, the type-map will create a private
+// hasher at the first call to Insert.
+//
+func (m *M) SetHasher(hasher Hasher) {
+	m.hasher = hasher
+}
+
+// Delete removes the entry with the given key, if any.
+// It returns true if the entry was found.
+//
+func (m *M) Delete(key types.Type) bool {
+	if m != nil && m.table != nil {
+		hash := m.hasher.Hash(key)
+		bucket := m.table[hash]
+		for i, e := range bucket {
+			if e.key != nil && types.IsIdentical(key, e.key) {
+				// We can't compact the bucket as it
+				// would disturb iterators.
+				bucket[i] = entry{}
+				m.length--
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// At returns the map entry for the given key.
+// The result is nil if the entry is not present.
+//
+func (m *M) At(key types.Type) interface{} {
+	if m != nil && m.table != nil {
+		for _, e := range m.table[m.hasher.Hash(key)] {
+			if e.key != nil && types.IsIdentical(key, e.key) {
+				return e.value
+			}
+		}
+	}
+	return nil
+}
+
+// Set sets the map entry for key to val,
+// and returns the previous entry, if any.
+func (m *M) Set(key types.Type, value interface{}) (prev interface{}) {
+	if m.table != nil {
+		hash := m.hasher.Hash(key)
+		bucket := m.table[hash]
+		var hole *entry
+		for i, e := range bucket {
+			if e.key == nil {
+				hole = &bucket[i]
+			} else if types.IsIdentical(key, e.key) {
+				prev = e.value
+				bucket[i].value = value
+				return
+			}
+		}
+
+		if hole != nil {
+			*hole = entry{key, value} // overwrite deleted entry
+		} else {
+			m.table[hash] = append(bucket, entry{key, value})
+		}
+	} else {
+		if m.hasher.memo == nil {
+			m.hasher = MakeHasher()
+		}
+		hash := m.hasher.Hash(key)
+		m.table = map[uint32][]entry{hash: {entry{key, value}}}
+	}
+
+	m.length++
+	return
+}
+
+// Len returns the number of map entries.
+func (m *M) Len() int {
+	if m != nil {
+		return m.length
+	}
+	return 0
+}
+
+// Iterate calls function f on each entry in the map in unspecified order.
+//
+// If f should mutate the map, Iterate provides the same guarantees as
+// Go maps: if f deletes a map entry that Iterate has not yet reached,
+// f will not be invoked for it, but if f inserts a map entry that
+// Iterate has not yet reached, whether or not f will be invoked for
+// it is unspecified.
+//
+func (m *M) Iterate(f func(key types.Type, value interface{})) {
+	if m != nil {
+		for _, bucket := range m.table {
+			for _, e := range bucket {
+				if e.key != nil {
+					f(e.key, e.value)
+				}
+			}
+		}
+	}
+}
+
+// Keys returns a new slice containing the set of map keys.
+// The order is unspecified.
+func (m *M) Keys() []types.Type {
+	keys := make([]types.Type, 0, m.Len())
+	m.Iterate(func(key types.Type, _ interface{}) {
+		keys = append(keys, key)
+	})
+	return keys
+}
+
+func (m *M) toString(values bool) string {
+	if m == nil {
+		return "{}"
+	}
+	var buf bytes.Buffer
+	fmt.Fprint(&buf, "{")
+	sep := ""
+	m.Iterate(func(key types.Type, value interface{}) {
+		fmt.Fprint(&buf, sep)
+		sep = ", "
+		fmt.Fprint(&buf, key)
+		if values {
+			fmt.Fprintf(&buf, ": %q", value)
+		}
+	})
+	fmt.Fprint(&buf, "}")
+	return buf.String()
+}
+
+// String returns a string representation of the map's entries.
+// Values are printed using fmt.Sprintf("%v", v).
+// Order is unspecified.
+//
+func (m *M) String() string {
+	return m.toString(true)
+}
+
+// KeysString returns a string representation of the map's key set.
+// Order is unspecified.
+//
+func (m *M) KeysString() string {
+	return m.toString(false)
+}
+
+////////////////////////////////////////////////////////////////////////
+// Hasher
+
+// A Hasher maps each type to its hash value.
+// For efficiency, a hasher uses memoization; thus its memory
+// footprint grows monotonically over time.
+// Hashers are not thread-safe.
+// Hashers have reference semantics.
+// Call MakeHasher to create a Hasher.
+type Hasher struct {
+	memo map[types.Type]uint32
+}
+
+// MakeHasher returns a new Hasher instance.
+func MakeHasher() Hasher {
+	return Hasher{make(map[types.Type]uint32)}
+}
+
+// Hash computes a hash value for the given type t such that
+// IsIdentical(t, t') => Hash(t) == Hash(t').
+func (h Hasher) Hash(t types.Type) uint32 {
+	hash, ok := h.memo[t]
+	if !ok {
+		hash = h.hashFor(t)
+		h.memo[t] = hash
+	}
+	return hash
+}
+
+// hashString computes the Fowler–Noll–Vo hash of s.
+func hashString(s string) uint32 {
+	var h uint32
+	for i := 0; i < len(s); i++ {
+		h ^= uint32(s[i])
+		h *= 16777619
+	}
+	return h
+}
+
+// hashFor computes the hash of t.
+func (h Hasher) hashFor(t types.Type) uint32 {
+	// See IsIdentical for rationale.
+	switch t := t.(type) {
+	case *types.Basic:
+		return uint32(t.Kind())
+
+	case *types.Array:
+		return 9043 + 2*uint32(t.Len()) + 3*h.Hash(t.Elem())
+
+	case *types.Slice:
+		return 9049 + 2*h.Hash(t.Elem())
+
+	case *types.Struct:
+		var hash uint32 = 9059
+		for i, n := 0, t.NumFields(); i < n; i++ {
+			f := t.Field(i)
+			if f.Anonymous() {
+				hash += 8861
+			}
+			hash += hashString(t.Tag(i))
+			hash += hashString(f.Name()) // (ignore f.Pkg)
+			hash += h.Hash(f.Type())
+		}
+		return hash
+
+	case *types.Pointer:
+		return 9067 + 2*h.Hash(t.Elem())
+
+	case *types.Signature:
+		var hash uint32 = 9091
+		if t.IsVariadic() {
+			hash *= 8863
+		}
+		return hash + 3*h.hashTuple(t.Params()) + 5*h.hashTuple(t.Results())
+
+	case *types.Interface:
+		var hash uint32 = 9103
+		for i, n := 0, t.NumMethods(); i < n; i++ {
+			// See go/types.identicalMethods for rationale.
+			// Method order is not significant.
+			// Ignore m.Pkg().
+			m := t.Method(i)
+			hash += 3*hashString(m.Name()) + 5*h.Hash(m.Type())
+		}
+		return hash
+
+	case *types.Map:
+		return 9109 + 2*h.Hash(t.Key()) + 3*h.Hash(t.Elem())
+
+	case *types.Chan:
+		return 9127 + 2*uint32(t.Dir()) + 3*h.Hash(t.Elem())
+
+	case *types.Named:
+		// Not safe with a copying GC; objects may move.
+		return uint32(uintptr(unsafe.Pointer(t.Obj())))
+	}
+	panic("unexpected type")
+}
+
+func (h Hasher) hashTuple(tuple *types.Tuple) uint32 {
+	// See go/types.identicalTypes for rationale.
+	n := tuple.Len()
+	var hash uint32 = 9137 + 2*uint32(n)
+	for i := 0; i < n; i++ {
+		hash += 3 * h.Hash(tuple.At(i).Type())
+	}
+	return hash
+}
diff --git a/go/types/typemap/typemap_test.go b/go/types/typemap/typemap_test.go
new file mode 100644
index 00000000000..295c82c6d93
--- /dev/null
+++ b/go/types/typemap/typemap_test.go
@@ -0,0 +1,170 @@
+package typemap_test
+
+// TODO(adonovan):
+// - test use of explicit hasher across two maps.
+// - test hashcodes are consistent with equals for a range of types
+//   (e.g. all types generated by type-checking some body of real code).
+
+import (
+	"code.google.com/p/go.tools/go/types"
+	"code.google.com/p/go.tools/go/types/typemap"
+	"go/ast"
+	"testing"
+)
+
+var (
+	tStr      = types.Typ[types.String]       // string
+	tPStr1    = types.NewPointer(tStr)        // *string
+	tPStr2    = types.NewPointer(tStr)        // *string, again
+	tInt      = types.Typ[types.Int]          // int
+	tChanInt1 = types.NewChan(ast.RECV, tInt) // <-chan int
+	tChanInt2 = types.NewChan(ast.RECV, tInt) // <-chan int, again
+)
+
+func checkEqualButNotIdentical(t *testing.T, x, y types.Type, comment string) {
+	if !types.IsIdentical(x, y) {
+		t.Errorf("%s: not equal: %s, %s", comment, x, y)
+	}
+	if x == y {
+		t.Errorf("%s: identical: %p, %p", comment, x, y)
+	}
+}
+
+func TestAxioms(t *testing.T) {
+	checkEqualButNotIdentical(t, tPStr1, tPStr2, "tPstr{1,2}")
+	checkEqualButNotIdentical(t, tChanInt1, tChanInt2, "tChanInt{1,2}")
+}
+
+func TestTypeMap(t *testing.T) {
+	var tmap *typemap.M
+
+	// All methods but Set are safe on on (*T)(nil).
+	tmap.Len()
+	tmap.At(tPStr1)
+	tmap.Delete(tPStr1)
+	tmap.KeysString()
+	tmap.String()
+
+	tmap = new(typemap.M)
+
+	// Length of empty map.
+	if l := tmap.Len(); l != 0 {
+		t.Errorf("Len() on empty typemap: got %d, want 0", l)
+	}
+	// At of missing key.
+	if v := tmap.At(tPStr1); v != nil {
+		t.Errorf("At() on empty typemap: got %v, want nil", v)
+	}
+	// Deletion of missing key.
+	if tmap.Delete(tPStr1) {
+		t.Errorf("Delete() on empty typemap: got true, want false")
+	}
+	// Set of new key.
+	if prev := tmap.Set(tPStr1, "*string"); prev != nil {
+		t.Errorf("Set() on empty map returned non-nil previous value %s", prev)
+	}
+
+	// Now: {*string: "*string"}
+
+	// Length of non-empty map.
+	if l := tmap.Len(); l != 1 {
+		t.Errorf("Len(): got %d, want 1", l)
+	}
+	// At via insertion key.
+	if v := tmap.At(tPStr1); v != "*string" {
+		t.Errorf("At(): got %q, want \"*string\"", v)
+	}
+	// At via equal key.
+	if v := tmap.At(tPStr2); v != "*string" {
+		t.Errorf("At(): got %q, want \"*string\"", v)
+	}
+	// Iteration over sole entry.
+	tmap.Iterate(func(key types.Type, value interface{}) {
+		if key != tPStr1 {
+			t.Errorf("Iterate: key: got %s, want %s", key, tPStr1)
+		}
+		if want := "*string"; value != want {
+			t.Errorf("Iterate: value: got %s, want %s", value, want)
+		}
+	})
+
+	// Setion with key equal to present one.
+	if prev := tmap.Set(tPStr2, "*string again"); prev != "*string" {
+		t.Errorf("Set() previous value: got %s, want \"*string\"", prev)
+	}
+
+	// Setion of another association.
+	if prev := tmap.Set(tChanInt1, "<-chan int"); prev != nil {
+		t.Errorf("Set() previous value: got %s, want nil", prev)
+	}
+
+	// Now: {*string: "*string again", <-chan int: "<-chan int"}
+
+	want1 := "{*string: \"*string again\", <-chan int: \"<-chan int\"}"
+	want2 := "{<-chan int: \"<-chan int\", *string: \"*string again\"}"
+	if s := tmap.String(); s != want1 && s != want2 {
+		t.Errorf("String(): got %s, want %s", s, want1)
+	}
+
+	want1 = "{*string, <-chan int}"
+	want2 = "{<-chan int, *string}"
+	if s := tmap.KeysString(); s != want1 && s != want2 {
+		t.Errorf("KeysString(): got %s, want %s", s, want1)
+	}
+
+	// Keys().
+	I := types.IsIdentical
+	switch k := tmap.Keys(); {
+	case I(k[0], tChanInt1) && I(k[1], tPStr1): // ok
+	case I(k[1], tChanInt1) && I(k[0], tPStr1): // ok
+	default:
+		t.Errorf("Keys(): got %v, want %s", k, want2)
+	}
+
+	if l := tmap.Len(); l != 2 {
+		t.Errorf("Len(): got %d, want 1", l)
+	}
+	// At via original key.
+	if v := tmap.At(tPStr1); v != "*string again" {
+		t.Errorf("At(): got %q, want \"*string again\"", v)
+	}
+	hamming := 1
+	tmap.Iterate(func(key types.Type, value interface{}) {
+		switch {
+		case I(key, tChanInt1):
+			hamming *= 2 // ok
+		case I(key, tPStr1):
+			hamming *= 3 // ok
+		}
+	})
+	if hamming != 6 {
+		t.Errorf("Iterate: hamming: got %d, want %d", hamming, 6)
+	}
+
+	if v := tmap.At(tChanInt2); v != "<-chan int" {
+		t.Errorf("At(): got %q, want \"<-chan int\"", v)
+	}
+	// Deletion with key equal to present one.
+	if !tmap.Delete(tChanInt2) {
+		t.Errorf("Delete() of existing key: got false, want true")
+	}
+
+	// Now: {*string: "*string again"}
+
+	if l := tmap.Len(); l != 1 {
+		t.Errorf("Len(): got %d, want 1", l)
+	}
+	// Deletion again.
+	if !tmap.Delete(tPStr2) {
+		t.Errorf("Delete() of existing key: got false, want true")
+	}
+
+	// Now: {}
+
+	if l := tmap.Len(); l != 0 {
+		t.Errorf("Len(): got %d, want %d", l, 0)
+	}
+	if s := tmap.String(); s != "{}" {
+		t.Errorf("Len(): got %q, want %q", s, "")
+	}
+}