diff --git a/go/types/check_test.go b/go/types/check_test.go
index f65ba49259..befeee21ce 100644
--- a/go/types/check_test.go
+++ b/go/types/check_test.go
@@ -48,6 +48,10 @@ var tests = [][]string{
 	{"testdata/errors.src"},
 	{"testdata/importdecl0a.src", "testdata/importdecl0b.src"},
 	{"testdata/cycles.src"},
+	{"testdata/cycles1.src"},
+	{"testdata/cycles2.src"},
+	{"testdata/cycles3.src"},
+	{"testdata/cycles4.src"},
 	{"testdata/decls0.src"},
 	{"testdata/decls1.src"},
 	{"testdata/decls2a.src", "testdata/decls2b.src"},
diff --git a/go/types/errors.go b/go/types/errors.go
index fe51597b9b..55a6c96cbe 100644
--- a/go/types/errors.go
+++ b/go/types/errors.go
@@ -14,7 +14,6 @@ import (
 	"strings"
 )
 
-// TODO(gri) eventually assert should disappear.
 func assert(p bool) {
 	if !p {
 		panic("assertion failed")
@@ -33,7 +32,7 @@ func (check *checker) formatMsg(format string, args []interface{}) string {
 		case operand:
 			panic("internal error: should always pass *operand")
 		case token.Pos:
-			args[i] = check.fset.Position(a)
+			args[i] = check.fset.Position(a).String()
 		case ast.Expr:
 			args[i] = exprString(a)
 		}
@@ -270,6 +269,17 @@ func writeType(buf *bytes.Buffer, typ Type) {
 		fmt.Fprintf(buf, "<type of %s>", t.name)
 
 	case *Interface:
+		// We write the source-level methods and embedded types rather
+		// than the actual method set since resolved method signatures
+		// may have non-printable cycles if parameters have anonymous
+		// interface types that (directly or indirectly) embed the
+		// current interface. For instance, consider the result type
+		// of m:
+		//
+		//     type T interface{
+		//         m() interface{ T }
+		//     }
+		//
 		buf.WriteString("interface{")
 		for i, m := range t.methods {
 			if i > 0 {
@@ -278,6 +288,12 @@ func writeType(buf *bytes.Buffer, typ Type) {
 			buf.WriteString(m.name)
 			writeSignature(buf, m.typ.(*Signature))
 		}
+		for i, typ := range t.types {
+			if i > 0 || len(t.methods) > 0 {
+				buf.WriteString("; ")
+			}
+			writeType(buf, typ)
+		}
 		buf.WriteByte('}')
 
 	case *Map:
diff --git a/go/types/eval_test.go b/go/types/eval_test.go
index 90a6deeeb3..41663185a7 100644
--- a/go/types/eval_test.go
+++ b/go/types/eval_test.go
@@ -58,7 +58,7 @@ func TestEvalBasic(t *testing.T) {
 }
 
 func TestEvalComposite(t *testing.T) {
-	for _, test := range testTypes {
+	for _, test := range independentTestTypes {
 		testEval(t, nil, nil, test.src, nil, test.str, "")
 	}
 }
diff --git a/go/types/gcimporter.go b/go/types/gcimporter.go
index 4648f43041..15c07b2119 100644
--- a/go/types/gcimporter.go
+++ b/go/types/gcimporter.go
@@ -16,6 +16,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"sort"
 	"strconv"
 	"strings"
 	"text/scanner"
@@ -633,7 +634,9 @@ func (p *gcParser) parseInterfaceType() Type {
 	}
 	p.expect('}')
 
+	sort.Sort(byUniqueMethodName(methods))
 	typ.methods = methods
+	typ.allMethods = methods // ok to share underlying array since we are not changing methods
 	return typ
 }
 
diff --git a/go/types/lookup.go b/go/types/lookup.go
index 398bb626ab..842a416180 100644
--- a/go/types/lookup.go
+++ b/go/types/lookup.go
@@ -177,7 +177,8 @@ func lookupFieldOrMethod(T Type, pkg *Package, name string) (obj Object, index [
 
 			case *Interface:
 				// look for a matching method
-				if i, m := lookupMethod(t.methods, pkg, name); m != nil {
+				// TODO(gri) t.allMethods is sorted - use binary search
+				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
 					assert(m.typ != nil)
 					index = concat(e.index, i)
 					if obj != nil || e.multiples {
@@ -252,8 +253,9 @@ func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType b
 	// TODO(gri) Consider using method sets here. Might be more efficient.
 
 	if ityp, _ := V.Underlying().(*Interface); ityp != nil {
-		for _, m := range T.methods {
-			_, obj := lookupMethod(ityp.methods, m.pkg, m.name)
+		// TODO(gri) allMethods is sorted - can do this more efficiently
+		for _, m := range T.allMethods {
+			_, obj := lookupMethod(ityp.allMethods, m.pkg, m.name)
 			switch {
 			case obj == nil:
 				if static {
@@ -267,7 +269,7 @@ func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType b
 	}
 
 	// A concrete type implements T if it implements all methods of T.
-	for _, m := range T.methods {
+	for _, m := range T.allMethods {
 		obj, _, indirect := lookupFieldOrMethod(V, m.pkg, m.name)
 		if obj == nil {
 			return m, false
diff --git a/go/types/methodset.go b/go/types/methodset.go
index 806cd15a4b..4ecd6f5daf 100644
--- a/go/types/methodset.go
+++ b/go/types/methodset.go
@@ -168,7 +168,7 @@ func NewMethodSet(T Type) *MethodSet {
 				}
 
 			case *Interface:
-				mset = mset.add(t.methods, e.index, true, e.multiples)
+				mset = mset.add(t.allMethods, e.index, true, e.multiples)
 			}
 		}
 
diff --git a/go/types/predicates.go b/go/types/predicates.go
index 82af6f800d..cde1ec4f8d 100644
--- a/go/types/predicates.go
+++ b/go/types/predicates.go
@@ -6,6 +6,8 @@
 
 package types
 
+import "sort"
+
 func isNamed(typ Type) bool {
 	if _, ok := typ.(*Basic); ok {
 		return ok
@@ -105,8 +107,22 @@ func hasNil(typ Type) bool {
 	return false
 }
 
-// IsIdentical returns true if x and y are identical.
+// IsIdentical reports whether x and y are identical.
 func IsIdentical(x, y Type) bool {
+	return isIdenticalInternal(x, y, nil)
+}
+
+// An ifacePair is a node in a stack of interface type pairs compared for identity.
+type ifacePair struct {
+	x, y *Interface
+	prev *ifacePair
+}
+
+func (p *ifacePair) identical(q *ifacePair) bool {
+	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
+}
+
+func isIdenticalInternal(x, y Type, p *ifacePair) bool {
 	if x == y {
 		return true
 	}
@@ -124,13 +140,13 @@ func IsIdentical(x, y Type) bool {
 		// Two array types are identical if they have identical element types
 		// and the same array length.
 		if y, ok := y.(*Array); ok {
-			return x.len == y.len && IsIdentical(x.elt, y.elt)
+			return x.len == y.len && isIdenticalInternal(x.elt, y.elt, p)
 		}
 
 	case *Slice:
 		// Two slice types are identical if they have identical element types.
 		if y, ok := y.(*Slice); ok {
-			return IsIdentical(x.elt, y.elt)
+			return isIdenticalInternal(x.elt, y.elt, p)
 		}
 
 	case *Struct:
@@ -145,7 +161,7 @@ func IsIdentical(x, y Type) bool {
 					if f.anonymous != g.anonymous ||
 						x.Tag(i) != y.Tag(i) ||
 						!f.sameId(g.pkg, g.name) ||
-						!IsIdentical(f.typ, g.typ) {
+						!isIdenticalInternal(f.typ, g.typ, p) {
 						return false
 					}
 				}
@@ -156,14 +172,24 @@ func IsIdentical(x, y Type) bool {
 	case *Pointer:
 		// Two pointer types are identical if they have identical base types.
 		if y, ok := y.(*Pointer); ok {
-			return IsIdentical(x.base, y.base)
+			return isIdenticalInternal(x.base, y.base, p)
 		}
 
 	case *Tuple:
 		// Two tuples types are identical if they have the same number of elements
 		// and corresponding elements have identical types.
 		if y, ok := y.(*Tuple); ok {
-			return identicalTuples(x, y)
+			if x.Len() == y.Len() {
+				if x != nil {
+					for i, v := range x.vars {
+						w := y.vars[i]
+						if !isIdenticalInternal(v.typ, w.typ, p) {
+							return false
+						}
+					}
+				}
+				return true
+			}
 		}
 
 	case *Signature:
@@ -173,8 +199,8 @@ func IsIdentical(x, y Type) bool {
 		// names are not required to match.
 		if y, ok := y.(*Signature); ok {
 			return x.isVariadic == y.isVariadic &&
-				identicalTuples(x.params, y.params) &&
-				identicalTuples(x.results, y.results)
+				isIdenticalInternal(x.params, y.params, p) &&
+				isIdenticalInternal(x.results, y.results, p)
 		}
 
 	case *Interface:
@@ -182,20 +208,63 @@ func IsIdentical(x, y Type) bool {
 		// the same names and identical function types. Lower-case method names from
 		// different packages are always different. The order of the methods is irrelevant.
 		if y, ok := y.(*Interface); ok {
-			return identicalMethods(x.methods, y.methods)
+			a := x.allMethods
+			b := y.allMethods
+			if len(a) == len(b) {
+				// Interface types are the only types where cycles can occur
+				// that are not "terminated" via named types; and such cycles
+				// can only be created via method parameter types that are
+				// anonymous interfaces (directly or indirectly) embedding
+				// the current interface. Example:
+				//
+				//    type T interface {
+				//        m() interface{T}
+				//    }
+				//
+				// If two such (differently named) interfaces are compared,
+				// endless recursion occurs if the cycle is not detected.
+				//
+				// If x and y were compared before, they must be equal
+				// (if they were not, the recursion would have stopped);
+				// search the ifacePair stack for the same pair.
+				//
+				// This is a quadratic algorithm, but in practice these stacks
+				// are extremely short (bounded by the nesting depth of interface
+				// type declarations that recur via parameter types, an extremely
+				// rare occurrence). An alternative implementation might use a
+				// "visited" map, but that is probably less efficient overall.
+				q := &ifacePair{x, y, p}
+				for p != nil {
+					if p.identical(q) {
+						return true // same pair was compared before
+					}
+					p = p.prev
+				}
+				if debug {
+					assert(sort.IsSorted(byUniqueMethodName(a)))
+					assert(sort.IsSorted(byUniqueMethodName(b)))
+				}
+				for i, f := range a {
+					g := b[i]
+					if f.Id() != g.Id() || !isIdenticalInternal(f.typ, g.typ, q) {
+						return false
+					}
+				}
+				return true
+			}
 		}
 
 	case *Map:
 		// Two map types are identical if they have identical key and value types.
 		if y, ok := y.(*Map); ok {
-			return IsIdentical(x.key, y.key) && IsIdentical(x.elt, y.elt)
+			return isIdenticalInternal(x.key, y.key, p) && isIdenticalInternal(x.elt, y.elt, p)
 		}
 
 	case *Chan:
 		// Two channel types are identical if they have identical value types
 		// and the same direction.
 		if y, ok := y.(*Chan); ok {
-			return x.dir == y.dir && IsIdentical(x.elt, y.elt)
+			return x.dir == y.dir && isIdenticalInternal(x.elt, y.elt, p)
 		}
 
 	case *Named:
@@ -204,53 +273,14 @@ func IsIdentical(x, y Type) bool {
 		if y, ok := y.(*Named); ok {
 			return x.obj == y.obj
 		}
+
+	default:
+		unreachable()
 	}
 
 	return false
 }
 
-// identicalTuples returns true if both tuples a and b have the
-// same length and corresponding elements have identical types.
-func identicalTuples(a, b *Tuple) bool {
-	if a.Len() != b.Len() {
-		return false
-	}
-	if a != nil {
-		for i, x := range a.vars {
-			y := b.vars[i]
-			if !IsIdentical(x.typ, y.typ) {
-				return false
-			}
-		}
-	}
-	return true
-}
-
-// identicalMethods returns true if both slices a and b have the
-// same length and corresponding entries have identical types.
-// TODO(gri) make this more efficient (e.g., sort them on completion)
-func identicalMethods(a, b []*Func) bool {
-	if len(a) != len(b) {
-		return false
-	}
-
-	m := make(map[string]*Func)
-	for _, x := range a {
-		key := x.Id()
-		assert(m[key] == nil) // method list must not have duplicate entries
-		m[key] = x
-	}
-
-	for _, y := range b {
-		key := y.Id()
-		if x := m[key]; x == nil || !IsIdentical(x.typ, y.typ) {
-			return false
-		}
-	}
-
-	return true
-}
-
 // defaultType returns the default "typed" type for an "untyped" type;
 // it returns the incoming type for all other types. The default type
 // for untyped nil is untyped nil.
diff --git a/go/types/resolver.go b/go/types/resolver.go
index 023bbdc57c..7476e4d14e 100644
--- a/go/types/resolver.go
+++ b/go/types/resolver.go
@@ -23,7 +23,7 @@ func (check *checker) reportAltDecl(obj Object) {
 	}
 }
 
-func (check *checker) declareObj(scope *Scope, id *ast.Ident, obj Object) {
+func (check *checker) declare(scope *Scope, id *ast.Ident, obj Object) {
 	if alt := scope.Insert(obj); alt != nil {
 		check.errorf(obj.Pos(), "%s redeclared in this block", obj.Name())
 		check.reportAltDecl(alt)
@@ -34,17 +34,6 @@ func (check *checker) declareObj(scope *Scope, id *ast.Ident, obj Object) {
 	}
 }
 
-func (check *checker) declareFld(oset *objset, id *ast.Ident, obj Object) {
-	if alt := oset.insert(obj); alt != nil {
-		check.errorf(obj.Pos(), "%s redeclared", obj.Name())
-		check.reportAltDecl(alt)
-		return
-	}
-	if id != nil {
-		check.recordObject(id, obj)
-	}
-}
-
 // A declInfo describes a package-level const, type, var, or func declaration.
 type declInfo struct {
 	file  *Scope        // scope of file containing this declaration
@@ -135,7 +124,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 			return
 		}
 
-		check.declareObj(pkg.scope, ident, obj)
+		check.declare(pkg.scope, ident, obj)
 		objList = append(objList, obj)
 		objMap[obj] = declInfo{fileScope, typ, init, nil}
 	}
@@ -227,7 +216,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 								if obj.IsExported() {
 									// Note: This will change each imported object's scope!
 									//       May be an issue for type aliases.
-									check.declareObj(fileScope, nil, obj)
+									check.declare(fileScope, nil, obj)
 									check.recordImplicit(s, obj)
 								}
 							}
@@ -241,7 +230,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 							posSet[imp] = s.Pos()
 						} else {
 							// declare imported package object in file scope
-							check.declareObj(fileScope, nil, obj)
+							check.declare(fileScope, nil, obj)
 						}
 
 					case *ast.ValueSpec:
@@ -326,7 +315,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 							// ok to continue
 						}
 					} else {
-						check.declareObj(pkg.scope, d.Name, obj)
+						check.declare(pkg.scope, d.Name, obj)
 					}
 				} else {
 					// Associate method with receiver base type name, if possible.
@@ -603,7 +592,7 @@ func (check *checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, cycleOk
 	//		C A
 	//	)
 	//
-	// When we declare obj = C, typ is the identifier A which is incomplete.
+	// When we declare object C, typ is the identifier A which is incomplete.
 	u := check.typ(typ, named, cycleOk)
 
 	// Determine the unnamed underlying type.
@@ -723,7 +712,7 @@ func (check *checker) declStmt(decl ast.Decl) {
 					check.arityMatch(s, last)
 
 					for i, name := range s.Names {
-						check.declareObj(check.topScope, name, lhs[i])
+						check.declare(check.topScope, name, lhs[i])
 					}
 
 				case token.VAR:
@@ -756,7 +745,7 @@ func (check *checker) declStmt(decl ast.Decl) {
 					check.arityMatch(s, nil)
 
 					for i, name := range s.Names {
-						check.declareObj(check.topScope, name, lhs[i])
+						check.declare(check.topScope, name, lhs[i])
 					}
 
 				default:
@@ -765,7 +754,7 @@ func (check *checker) declStmt(decl ast.Decl) {
 
 			case *ast.TypeSpec:
 				obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Name, nil)
-				check.declareObj(check.topScope, s.Name, obj)
+				check.declare(check.topScope, s.Name, obj)
 				check.typeDecl(obj, s.Type, nil, false)
 
 			default:
diff --git a/go/types/stdlib_test.go b/go/types/stdlib_test.go
index 413b4204e6..7763dca659 100644
--- a/go/types/stdlib_test.go
+++ b/go/types/stdlib_test.go
@@ -131,7 +131,6 @@ func TestStdfixed(t *testing.T) {
 		"bug200.go",                           // TODO(gri) complete duplicate checking in expr switches
 		"bug223.go", "bug413.go", "bug459.go", // TODO(gri) complete initialization checks
 		"bug248.go", "bug302.go", "bug369.go", // complex test instructions - ignore
-		"bug250.go",    // TODO(gri) fix recursive interfaces
 		"issue3924.go", // TODO(gri) && and || produce bool result (not untyped bool)
 		"issue4847.go", // TODO(gri) initialization cycle error not found
 	)
diff --git a/go/types/stmt.go b/go/types/stmt.go
index bca5ce804a..7d9a440eb1 100644
--- a/go/types/stmt.go
+++ b/go/types/stmt.go
@@ -441,7 +441,7 @@ func (check *checker) stmt(ctxt stmtContext, s ast.Stmt) {
 				// one; i.e., if any one of them is 'used', all of them are 'used'.
 				// Collect them for later analysis.
 				lhsVars = append(lhsVars, obj)
-				check.declareObj(check.topScope, nil, obj)
+				check.declare(check.topScope, nil, obj)
 				check.recordImplicit(clause, obj)
 			}
 			check.stmtList(inner, clause.Body)
@@ -592,7 +592,7 @@ func (check *checker) stmt(ctxt stmtContext, s ast.Stmt) {
 
 			// declare variables
 			for i, ident := range idents {
-				check.declareObj(check.topScope, ident, vars[i])
+				check.declare(check.topScope, ident, vars[i])
 			}
 		} else {
 			// ordinary assignment
diff --git a/go/types/testdata/cycles1.src b/go/types/testdata/cycles1.src
new file mode 100644
index 0000000000..ae2b38ebec
--- /dev/null
+++ b/go/types/testdata/cycles1.src
@@ -0,0 +1,77 @@
+// Copyright 2013 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.
+
+package p
+
+type (
+	A interface {
+		a() interface {
+			ABC1
+		}
+	}
+	B interface {
+		b() interface {
+			ABC2
+		}
+	}
+	C interface {
+		c() interface {
+			ABC3
+		}
+	}
+
+	AB interface {
+		A
+		B
+	}
+	BC interface {
+		B
+		C
+	}
+
+	ABC1 interface {
+		A
+		B
+		C
+	}
+	ABC2 interface {
+		AB
+		C
+	}
+	ABC3 interface {
+		A
+		BC
+	}
+)
+
+var (
+	x1 ABC1
+	x2 ABC2
+	x3 ABC3
+)
+
+func _() {
+	// all types have the same method set
+	x1 = x2
+	x2 = x1
+
+	x1 = x3
+	x3 = x1
+
+	x2 = x3
+	x3 = x2
+
+	// all methods return the same type again
+	x1 = x1.a()
+	x1 = x1.b()
+	x1 = x1.c()
+
+	x2 = x2.a()
+	x2 = x2.b()
+	x2 = x2.c()
+
+	x3 = x3.a()
+	x3 = x3.b()
+	x3 = x3.c()
+}
diff --git a/go/types/testdata/cycles2.src b/go/types/testdata/cycles2.src
new file mode 100644
index 0000000000..3b9f473d07
--- /dev/null
+++ b/go/types/testdata/cycles2.src
@@ -0,0 +1,66 @@
+// Copyright 2013 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.
+
+package p
+
+// Test case for issue 5090
+
+type t interface {
+	f(u)
+}
+
+type u interface {
+	t
+}
+
+func _() {
+	var t t
+	var u u
+
+	t.f(t)
+	t.f(u)
+	
+	u.f(t)
+	u.f(u)
+}
+
+
+// Test case for issue 6589.
+
+type A interface {
+	a() interface {
+		AB
+	}
+}
+
+type B interface {
+	a() interface {
+		AB
+	}
+}
+
+type AB interface {
+	a() interface {
+		A
+		B /* ERROR a redeclared */
+	}
+	b() interface {
+		A
+		B /* ERROR a redeclared */
+	}
+}
+
+var x AB
+var y interface {
+	A
+	B /* ERROR a redeclared */
+}
+var _ = x /* ERROR cannot compare */ == y
+
+
+// Test case for issue 6638.
+
+type T /* ERROR cycle */ interface {
+	m() [T /* ERROR no field or method */ (nil).m()[0]]int
+}
diff --git a/go/types/testdata/cycles3.src b/go/types/testdata/cycles3.src
new file mode 100644
index 0000000000..cf3e911149
--- /dev/null
+++ b/go/types/testdata/cycles3.src
@@ -0,0 +1,60 @@
+// Copyright 2013 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.
+
+package p
+
+import "unsafe"
+
+var (
+	_ A = A(nil).a().b().c().d().e().f()
+	_ A = A(nil).b().c().d().e().f()
+	_ A = A(nil).c().d().e().f()
+	_ A = A(nil).d().e().f()
+	_ A = A(nil).e().f()
+	_ A = A(nil).f()
+	_ A = A(nil)
+)
+
+type (
+	A interface {
+		a() B
+		B
+	}
+
+	B interface {
+		b() C
+		C
+	}
+
+	C interface {
+		c() D
+		D
+	}
+
+	D interface {
+		d() E
+		E
+	}
+
+	E interface {
+		e() F
+		F
+	}
+
+	F interface {
+		f() A
+	}
+)
+
+type (
+	U /* ERROR illegal cycle */ interface {
+		V
+	}
+
+	V interface {
+		v() [unsafe.Sizeof(u)]int
+	}
+)
+
+var u U
diff --git a/go/types/testdata/cycles4.src b/go/types/testdata/cycles4.src
new file mode 100644
index 0000000000..528f205777
--- /dev/null
+++ b/go/types/testdata/cycles4.src
@@ -0,0 +1,68 @@
+// Copyright 2013 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.
+
+package p
+
+// Check that all methods of T are collected before
+// determining the result type of m (which embeds
+// all methods of T).
+
+type T interface {
+	m() interface {T}
+	E
+}
+
+var _ = T.m(nil).m().e()
+
+type E interface {
+	e() int
+}
+
+// Check that unresolved forward chains are followed
+// (see also comment in resolver.go, checker.typeDecl).
+
+var _ = C.m(nil).m().e()
+
+type A B
+
+type B interface {
+	m() interface{C}
+	E
+}
+
+type C A
+
+// Check that interface type comparison for identity
+// does not recur endlessly.
+
+type T1 interface {
+	m() interface{T1}
+}
+
+type T2 interface {
+	m() interface{T2}
+}
+
+func _(x T1, y T2) {
+	// Checking for assignability of interfaces must check
+	// if all methods of x are present in y, and that they
+	// have identical signatures. The signatures recur via
+	// the result type, which is an interface that embeds
+	// a single method m that refers to the very interface
+	// that contains it. This requires cycle detection in
+	// identity checks for interface types.
+	x = y
+}
+
+type T3 interface {
+	m() interface{T4}
+}
+
+type T4 interface {
+	m() interface{T3}
+}
+
+func _(x T1, y T3) {
+	x = y
+}
diff --git a/go/types/types.go b/go/types/types.go
index 262272cd1a..8b54a7fc03 100644
--- a/go/types/types.go
+++ b/go/types/types.go
@@ -4,7 +4,10 @@
 
 package types
 
-import "go/ast"
+import (
+	"go/ast"
+	"sort"
+)
 
 // TODO(gri) Revisit factory functions - make sure they have all relevant parameters.
 
@@ -238,20 +241,25 @@ func (s *Signature) IsVariadic() bool { return s.isVariadic }
 
 // An Interface represents an interface type.
 type Interface struct {
-	methods []*Func         // methods declared with or embedded in this interface
-	mset    cachedMethodSet // method set for interface, lazily initialized
+	methods []*Func  // explicitly declared methods
+	types   []*Named // explicitly embedded types
+
+	allMethods []*Func         // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
+	mset       cachedMethodSet // method set for interface, lazily initialized
 }
 
 // NewInterface returns a new interface for the given methods.
 func NewInterface(methods []*Func) *Interface {
-	return &Interface{methods: methods}
+	// TODO(gri) should provide receiver to all methods
+	sort.Sort(byUniqueMethodName(methods))
+	return &Interface{methods: methods, allMethods: methods}
 }
 
 // NumMethods returns the number of methods of interface t.
-func (t *Interface) NumMethods() int { return len(t.methods) }
+func (t *Interface) NumMethods() int { return len(t.allMethods) }
 
 // Method returns the i'th method of interface t for 0 <= i < t.NumMethods().
-func (t *Interface) Method(i int) *Func { return t.methods[i] }
+func (t *Interface) Method(i int) *Func { return t.allMethods[i] }
 
 // A Map represents a map type.
 type Map struct {
diff --git a/go/types/types_test.go b/go/types/types_test.go
index 548b4006bd..ca3054883e 100644
--- a/go/types/types_test.go
+++ b/go/types/types_test.go
@@ -33,7 +33,8 @@ func dup(s string) testEntry {
 	return testEntry{s, s}
 }
 
-var testTypes = []testEntry{
+// types that don't depend on any other type declarations
+var independentTestTypes = []testEntry{
 	// basic types
 	dup("int"),
 	dup("float32"),
@@ -89,8 +90,7 @@ var testTypes = []testEntry{
 	// interfaces
 	dup("interface{}"),
 	dup("interface{m()}"),
-	dup(`interface{m(int) float32; String() string}`),
-	// TODO(gri) add test for interface w/ anonymous field
+	dup(`interface{String() string; m(int) float32}`),
 
 	// maps
 	dup("map[string]int"),
@@ -102,9 +102,21 @@ var testTypes = []testEntry{
 	dup("<-chan []func() int"),
 }
 
+// types that depend on other type declarations (src in TestTypes)
+var dependentTestTypes = []testEntry{
+	// interfaces
+	dup(`interface{io.Reader; io.Writer}`),
+	dup(`interface{m() int; io.Writer}`),
+	{`interface{m() interface{T}}`, `interface{m() interface{p.T}}`},
+}
+
 func TestTypes(t *testing.T) {
-	for _, test := range testTypes {
-		src := "package p; type T " + test.src
+	var tests []testEntry
+	tests = append(tests, independentTestTypes...)
+	tests = append(tests, dependentTestTypes...)
+
+	for _, test := range tests {
+		src := `package p; import "io"; type _ io.Writer; type T ` + test.src
 		pkg, err := makePkg(t, src)
 		if err != nil {
 			t.Errorf("%s: %s", src, err)
diff --git a/go/types/typexpr.go b/go/types/typexpr.go
index 5265490ea3..865f7af85b 100644
--- a/go/types/typexpr.go
+++ b/go/types/typexpr.go
@@ -9,6 +9,7 @@ package types
 import (
 	"go/ast"
 	"go/token"
+	"sort"
 	"strconv"
 
 	"code.google.com/p/go.tools/go/exact"
@@ -67,8 +68,9 @@ func (check *checker) ident(x *operand, e *ast.Ident, def *Named, cycleOk bool)
 			}
 			x.val = check.iota
 		} else {
-			x.val = obj.val // may be nil if we don't know the constant value
+			x.val = obj.val
 		}
+		assert(x.val != nil)
 		x.mode = constant
 
 	case *TypeName:
@@ -166,6 +168,7 @@ func (check *checker) funcType(recv *ast.FieldList, ftyp *ast.FuncType, def *Nam
 				if T.obj.pkg != check.pkg {
 					err = "type not defined in this package"
 				} else {
+					// TODO(gri) This is not correct if the underlying type is unknown yet.
 					switch u := T.underlying.(type) {
 					case *Basic:
 						// unsafe.Pointer is treated like a regular pointer
@@ -297,15 +300,7 @@ func (check *checker) typInternal(e ast.Expr, def *Named, cycleOk bool) Type {
 		return check.funcType(nil, e, def)
 
 	case *ast.InterfaceType:
-		typ := new(Interface)
-		var recv Type = typ
-		if def != nil {
-			def.underlying = typ
-			recv = def // use named receiver type if available
-		}
-
-		typ.methods = check.collectMethods(recv, e.Methods, cycleOk)
-		return typ
+		return check.interfaceType(e, def, cycleOk)
 
 	case *ast.MapType:
 		typ := new(Map)
@@ -319,6 +314,7 @@ func (check *checker) typInternal(e ast.Expr, def *Named, cycleOk bool) Type {
 		// spec: "The comparison operators == and != must be fully defined
 		// for operands of the key type; thus the key type must not be a
 		// function, map, or slice."
+		// TODO(gri) if the key type is not fully defined yet, this test will be incorrect
 		if !isComparable(typ.key) {
 			check.errorf(e.Key.Pos(), "invalid map key type %s", typ.key)
 			// ok to continue
@@ -385,13 +381,13 @@ func (check *checker) collectParams(scope *Scope, list *ast.FieldList, variadicO
 			}
 		}
 		typ := check.typ(ftype, nil, true)
-		// the parser ensures that f.Tag is nil and we don't
-		// care if a constructed AST contains a non-nil tag
+		// The parser ensures that f.Tag is nil and we don't
+		// care if a constructed AST contains a non-nil tag.
 		if len(field.Names) > 0 {
 			// named parameter
 			for _, name := range field.Names {
 				par := NewParam(name.Pos(), check.pkg, name.Name, typ)
-				check.declareObj(scope, name, par)
+				check.declare(scope, name, par)
 				params = append(params, par)
 			}
 		} else {
@@ -411,57 +407,152 @@ func (check *checker) collectParams(scope *Scope, list *ast.FieldList, variadicO
 	return
 }
 
-func (check *checker) collectMethods(recv Type, list *ast.FieldList, cycleOk bool) (methods []*Func) {
-	if list == nil {
-		return nil
+func (check *checker) declareInSet(oset *objset, pos token.Pos, id *ast.Ident, obj Object) bool {
+	if alt := oset.insert(obj); alt != nil {
+		check.errorf(pos, "%s redeclared", obj.Name())
+		check.reportAltDecl(alt)
+		return false
+	}
+	if id != nil {
+		check.recordObject(id, obj)
+	}
+	return true
+}
+
+func (check *checker) interfaceType(ityp *ast.InterfaceType, def *Named, cycleOk bool) *Interface {
+	iface := new(Interface)
+	if def != nil {
+		def.underlying = iface
 	}
 
-	var mset objset
+	// empty interface: common case
+	if ityp.Methods == nil {
+		return iface
+	}
 
-	for _, f := range list.List {
-		typ := check.typ(f.Type, nil, cycleOk)
-		// the parser ensures that f.Tag is nil and we don't
-		// care if a constructed AST contains a non-nil tag
+	// The parser ensures that field tags are nil and we don't
+	// care if a constructed AST contains non-nil tags.
+
+	// Phase 1: Collect explicitly declared methods, the corresponding
+	//          signature (AST) expressions, and the list of embedded
+	//          type (AST) expressions. Do not resolve signatures or
+	//          embedded types yet to avoid cycles referring to this
+	//          interface.
+
+	var (
+		mset       objset
+		signatures []ast.Expr // list of corresponding method signatures
+		embedded   []ast.Expr // list of embedded types
+	)
+	for _, f := range ityp.Methods.List {
 		if len(f.Names) > 0 {
-			// methods (the parser ensures that there's only one
-			// and we don't care if a constructed AST has more)
-			sig, _ := typ.(*Signature)
-			if sig == nil {
-				check.invalidAST(f.Type.Pos(), "%s is not a method signature", typ)
-				continue
-			}
-			sig.recv = NewVar(token.NoPos, check.pkg, "", recv)
-			for _, name := range f.Names {
-				m := NewFunc(name.Pos(), check.pkg, name.Name, sig)
-				check.declareFld(&mset, name, m)
-				methods = append(methods, m)
+			// The parser ensures that there's only one method
+			// and we don't care if a constructed AST has more.
+			name := f.Names[0]
+			pos := name.Pos()
+			// Don't type-check signature yet - use an
+			// empty signature now and update it later.
+			m := NewFunc(pos, check.pkg, name.Name, new(Signature))
+			if check.declareInSet(&mset, pos, name, m) {
+				iface.methods = append(iface.methods, m)
+				iface.allMethods = append(iface.allMethods, m)
+				signatures = append(signatures, f.Type)
 			}
 		} else {
-			// embedded interface
-			switch t := typ.Underlying().(type) {
-			case nil:
-				// The underlying type is in the process of being defined
-				// but we need it in order to complete this type. For now
-				// complain with an "unimplemented" error. This requires
-				// a bit more work.
-				// TODO(gri) finish this.
-				check.errorf(f.Type.Pos(), "reference to incomplete type %s - unimplemented", f.Type)
-			case *Interface:
-				for _, m := range t.methods {
-					check.declareFld(&mset, nil, m)
-					methods = append(methods, m)
-				}
-			default:
-				if t != Typ[Invalid] {
-					check.errorf(f.Type.Pos(), "%s is not an interface type", typ)
-				}
+			// embedded type
+			embedded = append(embedded, f.Type)
+		}
+	}
+
+	// Phase 2: Resolve embedded interfaces. Because an interface must not
+	//          embed itself (directly or indirectly), each embedded interface
+	//          can be fully resolved without depending on any method of this
+	//          interface (if there is a cycle or another error, the embedded
+	//          type resolves to an invalid type and is ignored).
+	//          In particular, the list of methods for each embedded interface
+	//          must be complete (it cannot depend on this interface), and so
+	//          those methods can be added to the list of all methods of this
+	//          interface.
+
+	for _, e := range embedded {
+		pos := e.Pos()
+		typ := check.typ(e, nil, cycleOk)
+		if typ == Typ[Invalid] {
+			continue
+		}
+		named, _ := typ.(*Named)
+		if named == nil {
+			check.invalidAST(pos, "%s is not named type", typ)
+			continue
+		}
+		// determine underlying (possibly incomplete) type
+		// by following its forward chain
+		// TODO(gri) should this be part of Underlying()?
+		u := named.underlying
+		for {
+			n, _ := u.(*Named)
+			if n == nil {
+				break
+			}
+			u = n.underlying
+		}
+		if u == Typ[Invalid] {
+			continue
+		}
+		embed, _ := u.(*Interface)
+		if embed == nil {
+			check.errorf(pos, "%s is not an interface", named)
+			continue
+		}
+		iface.types = append(iface.types, named)
+		// collect embedded methods
+		for _, m := range embed.allMethods {
+			if check.declareInSet(&mset, pos, nil, m) {
+				iface.allMethods = append(iface.allMethods, m)
 			}
 		}
 	}
 
-	return
+	// Phase 3: At this point all methods have been collected for this interface.
+	//          It is now safe to type-check the signatures of all explicitly
+	//          declared methods, even if they refer to this interface via a cycle
+	//          and embed the methods of this interface in a parameter of interface
+	//          type.
+
+	// determine receiver type
+	var recv Type = iface
+	if def != nil {
+		def.underlying = iface
+		recv = def // use named receiver type if available
+	}
+
+	for i, m := range iface.methods {
+		expr := signatures[i]
+		typ := check.typ(expr, nil, true)
+		if typ == Typ[Invalid] {
+			continue // keep method with empty method signature
+		}
+		sig, _ := typ.(*Signature)
+		if sig == nil {
+			check.invalidAST(expr.Pos(), "%s is not a method signature", typ)
+			continue // keep method with empty method signature
+		}
+		sig.recv = NewVar(m.pos, check.pkg, "", recv)
+		*m.typ.(*Signature) = *sig // update signature (don't replace it!)
+	}
+
+	sort.Sort(byUniqueMethodName(iface.allMethods))
+
+	return iface
 }
 
+// byUniqueMethodName method lists can be sorted by their unique method names.
+type byUniqueMethodName []*Func
+
+func (a byUniqueMethodName) Len() int           { return len(a) }
+func (a byUniqueMethodName) Less(i, j int) bool { return a[i].Id() < a[j].Id() }
+func (a byUniqueMethodName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
 func (check *checker) tag(t *ast.BasicLit) string {
 	if t != nil {
 		if t.Kind == token.STRING {
@@ -492,8 +583,9 @@ func (check *checker) collectFields(list *ast.FieldList, cycleOk bool) (fields [
 		}
 
 		fld := NewField(pos, check.pkg, name, typ, anonymous)
-		check.declareFld(&fset, ident, fld)
-		fields = append(fields, fld)
+		if check.declareInSet(&fset, pos, ident, fld) {
+			fields = append(fields, fld)
+		}
 	}
 
 	for _, f := range list.List {
diff --git a/go/types/universe.go b/go/types/universe.go
index 049f7d6dbe..63a6be11f1 100644
--- a/go/types/universe.go
+++ b/go/types/universe.go
@@ -67,7 +67,7 @@ func defPredeclaredTypes() {
 	res := NewVar(token.NoPos, nil, "", Typ[String])
 	sig := &Signature{results: NewTuple(res)}
 	err := NewFunc(token.NoPos, nil, "Error", sig)
-	typ := &Named{underlying: &Interface{methods: []*Func{err}}, complete: true}
+	typ := &Named{underlying: NewInterface([]*Func{err}), complete: true}
 	sig.recv = NewVar(token.NoPos, nil, "", typ)
 	def(NewTypeName(token.NoPos, nil, "error", typ))
 }