diff --git a/go/types/assignments.go b/go/types/assignments.go
index f045acb4..de4b28fe 100644
--- a/go/types/assignments.go
+++ b/go/types/assignments.go
@@ -264,7 +264,7 @@ func (check *checker) shortVarDecl(lhs, rhs []ast.Expr) {
 			// Use the correct obj if the ident is redeclared. The
 			// variable's scope starts after the declaration; so we
 			// must use Scope.Lookup here and call Scope.Insert later.
-			if alt := scope.Lookup(nil, ident.Name); alt != nil {
+			if alt := scope.Lookup(ident.Name); alt != nil {
 				// redeclared object must be a variable
 				if alt, _ := alt.(*Var); alt != nil {
 					obj = alt
diff --git a/go/types/call.go b/go/types/call.go
index aa3da728..7e3786fd 100644
--- a/go/types/call.go
+++ b/go/types/call.go
@@ -171,7 +171,7 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 	if ident, ok := e.X.(*ast.Ident); ok {
 		if pkg, ok := check.topScope.LookupParent(ident.Name).(*Package); ok {
 			check.recordObject(ident, pkg)
-			exp := pkg.scope.Lookup(nil, sel)
+			exp := pkg.scope.Lookup(sel)
 			if exp == nil {
 				check.errorf(e.Pos(), "%s not declared by package %s", sel, ident)
 				goto Error
diff --git a/go/types/gcimporter.go b/go/types/gcimporter.go
index ab249d08..cbd6f3e5 100644
--- a/go/types/gcimporter.go
+++ b/go/types/gcimporter.go
@@ -207,7 +207,7 @@ func declConst(pkg *Package, name string) *Const {
 	// the constant may have been imported before - if it exists
 	// already in the respective scope, return that constant
 	scope := pkg.scope
-	if obj := scope.Lookup(nil, name); obj != nil {
+	if obj := scope.Lookup(name); obj != nil {
 		return obj.(*Const)
 	}
 	// otherwise create a new constant and insert it into the scope
@@ -218,7 +218,7 @@ func declConst(pkg *Package, name string) *Const {
 
 func declTypeName(pkg *Package, name string) *TypeName {
 	scope := pkg.scope
-	if obj := scope.Lookup(nil, name); obj != nil {
+	if obj := scope.Lookup(name); obj != nil {
 		return obj.(*TypeName)
 	}
 	obj := NewTypeName(token.NoPos, pkg, name, nil)
@@ -231,7 +231,7 @@ func declTypeName(pkg *Package, name string) *TypeName {
 
 func declVar(pkg *Package, name string) *Var {
 	scope := pkg.scope
-	if obj := scope.Lookup(nil, name); obj != nil {
+	if obj := scope.Lookup(name); obj != nil {
 		return obj.(*Var)
 	}
 	obj := NewVar(token.NoPos, pkg, name, nil)
@@ -241,7 +241,7 @@ func declVar(pkg *Package, name string) *Var {
 
 func declFunc(pkg *Package, name string) *Func {
 	scope := pkg.scope
-	if obj := scope.Lookup(nil, name); obj != nil {
+	if obj := scope.Lookup(name); obj != nil {
 		return obj.(*Func)
 	}
 	obj := NewFunc(token.NoPos, pkg, name, nil)
@@ -390,7 +390,7 @@ func (p *gcParser) parseExportedName() (pkg *Package, name string) {
 //
 func (p *gcParser) parseBasicType() Type {
 	id := p.expect(scanner.Ident)
-	obj := Universe.Lookup(nil, id)
+	obj := Universe.Lookup(id)
 	if obj, ok := obj.(*TypeName); ok {
 		return obj.typ
 	}
diff --git a/go/types/gcimporter_test.go b/go/types/gcimporter_test.go
index ad9e8ddf..bc0378a8 100644
--- a/go/types/gcimporter_test.go
+++ b/go/types/gcimporter_test.go
@@ -146,7 +146,7 @@ func TestGcImportedTypes(t *testing.T) {
 			continue
 		}
 
-		obj := pkg.scope.Lookup(nil, objName)
+		obj := pkg.scope.Lookup(objName)
 
 		// TODO(gri) should define an accessor on Object
 		var kind ast.ObjKind
diff --git a/go/types/objset.go b/go/types/objset.go
new file mode 100644
index 00000000..afb92c10
--- /dev/null
+++ b/go/types/objset.go
@@ -0,0 +1,36 @@
+// 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.
+
+// This file implements objsets.
+//
+// An objset is similar to a Scope but objset elements
+// are identified by their unique id, instead of their
+// object name.
+
+package types
+
+// An objset is a set of objects identified by their unique id.
+// The zero value for objset is a ready-to-use empty objset.
+type objset struct {
+	objmap map[string]Object // allocated lazily
+}
+
+// insert attempts to insert an object obj into objset s.
+// If s already contains an alternative object alt with
+// the same name, insert leaves s unchanged and returns alt.
+// Otherwise it inserts obj and returns nil.
+// The object name must not be blank _.
+func (s *objset) insert(obj Object) (alt Object) {
+	name := obj.Name()
+	assert(name != "_")
+	id := Id(obj.Pkg(), name)
+	if alt := s.objmap[id]; alt != nil {
+		return alt
+	}
+	if s.objmap == nil {
+		s.objmap = make(map[string]Object)
+	}
+	s.objmap[id] = obj
+	return nil
+}
diff --git a/go/types/resolver.go b/go/types/resolver.go
index ceee0884..55733510 100644
--- a/go/types/resolver.go
+++ b/go/types/resolver.go
@@ -23,14 +23,29 @@ func (check *checker) reportAltDecl(obj Object) {
 	}
 }
 
-func (check *checker) declare(scope *Scope, id *ast.Ident, obj Object) {
+func (check *checker) declareObj(scope *Scope, id *ast.Ident, obj Object) {
 	if obj.Name() == "_" {
 		// blank identifiers are not declared
 		obj.setParent(scope)
+		obj = nil
 	} else if alt := scope.Insert(obj); alt != nil {
 		check.errorf(obj.Pos(), "%s redeclared in this block", obj.Name())
 		check.reportAltDecl(alt)
-		obj = nil // for callIdent below
+		obj = nil
+	}
+	if id != nil {
+		check.recordObject(id, obj)
+	}
+}
+
+func (check *checker) declareFld(oset *objset, id *ast.Ident, obj Object) {
+	if obj.Name() == "_" {
+		// blank identifiers are not declared
+		obj = nil
+	} else if alt := oset.insert(obj); alt != nil {
+		check.errorf(obj.Pos(), "%s redeclared", obj.Name())
+		check.reportAltDecl(alt)
+		obj = nil
 	}
 	if id != nil {
 		check.recordObject(id, obj)
@@ -128,7 +143,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 			return
 		}
 
-		check.declare(pkg.scope, ident, obj)
+		check.declareObj(pkg.scope, ident, obj)
 		objList = append(objList, obj)
 		objMap[obj] = declInfo{scope, typ, init, nil}
 	}
@@ -195,13 +210,13 @@ func (check *checker) resolveFiles(files []*ast.File) {
 								if obj.IsExported() {
 									// Note: This will change each imported object's scope!
 									//       May be an issue for types aliases.
-									check.declare(scope, nil, obj)
+									check.declareObj(scope, nil, obj)
 									check.recordImplicit(s, obj)
 								}
 							}
 						} else {
 							// declare imported package object in file scope
-							check.declare(scope, nil, imp2)
+							check.declareObj(scope, nil, imp2)
 						}
 
 					case *ast.ValueSpec:
@@ -281,7 +296,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 						obj.parent = pkg.scope
 						check.recordObject(d.Name, obj)
 					} else {
-						check.declare(pkg.scope, d.Name, obj)
+						check.declareObj(pkg.scope, d.Name, obj)
 					}
 				} else {
 					// Associate method with receiver base type name, if possible.
@@ -313,7 +328,7 @@ func (check *checker) resolveFiles(files []*ast.File) {
 
 	for _, scope := range scopes {
 		for _, obj := range scope.entries {
-			if alt := pkg.scope.Lookup(nil, obj.Name()); alt != nil {
+			if alt := pkg.scope.Lookup(obj.Name()); alt != nil {
 				check.errorf(alt.Pos(), "%s already declared in this file through import of package %s", obj.Name(), obj.Pkg().Name())
 			}
 		}
@@ -503,41 +518,39 @@ func (check *checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, cycleOk
 
 	// spec: "For a base type, the non-blank names of methods bound
 	// to it must be unique."
-	// => use a scope to determine redeclarations
-	scope := NewScope(nil)
+	// => use an objset to determine redeclarations
+	var mset objset
 
 	// spec: "If the base type is a struct type, the non-blank method
 	// and field names must be distinct."
-	// => pre-populate the scope to find conflicts
+	// => pre-populate the objset to find conflicts
+	// TODO(gri) consider keeping the objset with the struct instead
 	if t, _ := named.underlying.(*Struct); t != nil {
 		for _, fld := range t.fields {
 			if fld.name != "_" {
-				scope.Insert(fld)
+				assert(mset.insert(fld) == nil)
 			}
 		}
 	}
 
 	// check each method
 	for _, m := range methods {
-		assert(m.name != "_") // _ methods were excluded before
-		mdecl := check.objMap[m]
-		alt := scope.Insert(m)
-		m.parent = mdecl.file // correct parent scope (scope.Insert used scope)
+		ident := check.objMap[m].fdecl.Name
 
-		if alt != nil {
-			switch alt := alt.(type) {
+		if alt := mset.insert(m); alt != nil {
+			switch alt.(type) {
 			case *Var:
 				check.errorf(m.pos, "field and method with the same name %s", m.name)
-				check.reportAltDecl(alt)
-				m = nil
 			case *Func:
 				check.errorf(m.pos, "method %s already declared for %s", m.name, named)
-				check.reportAltDecl(alt)
-				m = nil
+			default:
+				unreachable()
 			}
+			check.reportAltDecl(alt)
+			m = nil
 		}
 
-		check.recordObject(mdecl.fdecl.Name, m)
+		check.recordObject(ident, m)
 
 		// If the method is valid, type-check its signature,
 		// and collect it with the named base type.
@@ -604,7 +617,7 @@ func (check *checker) declStmt(decl ast.Decl) {
 					check.arityMatch(s, last)
 
 					for i, name := range s.Names {
-						check.declare(check.topScope, name, lhs[i])
+						check.declareObj(check.topScope, name, lhs[i])
 					}
 
 				case token.VAR:
@@ -637,7 +650,7 @@ func (check *checker) declStmt(decl ast.Decl) {
 					check.arityMatch(s, nil)
 
 					for i, name := range s.Names {
-						check.declare(check.topScope, name, lhs[i])
+						check.declareObj(check.topScope, name, lhs[i])
 					}
 
 				default:
@@ -646,7 +659,7 @@ func (check *checker) declStmt(decl ast.Decl) {
 
 			case *ast.TypeSpec:
 				obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Name, nil)
-				check.declare(check.topScope, s.Name, obj)
+				check.declareObj(check.topScope, s.Name, obj)
 				check.typeDecl(obj, s.Type, nil, false)
 
 			default:
diff --git a/go/types/scope.go b/go/types/scope.go
index faafa1c1..062dbd16 100644
--- a/go/types/scope.go
+++ b/go/types/scope.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.
 
+// This file implements Scopes.
+
 package types
 
 import (
@@ -16,15 +18,16 @@ import (
 //           objects can use that information for better printing.
 
 // A Scope maintains a set of objects and links to its containing
-// (parent) and contained (children) scopes.
-// Objects may be inserted and looked up by name, or by package path
-// and name. A nil *Scope acts like an empty scope for operations that
-// do not modify the scope or access a scope's parent scope.
+// (parent) and contained (children) scopes. Objects may be inserted
+// and looked up by name. The zero value for Scope is a ready-to-use
+// empty scope.
 type Scope struct {
 	parent   *Scope
 	children []*Scope
-	entries  []Object
 	node     ast.Node
+
+	entries []Object
+	objmap  map[string]Object // lazily allocated for large scopes
 }
 
 // NewScope returns a new, empty scope contained in the given parent
@@ -60,74 +63,39 @@ func (s *Scope) Parent() *Scope { return s.parent }
 func (s *Scope) Node() ast.Node { return s.node }
 
 // NumEntries() returns the number of scope entries.
-// If s == nil, the result is 0.
-func (s *Scope) NumEntries() int {
-	if s == nil {
-		return 0 // empty scope
-	}
-	return len(s.entries)
-}
+func (s *Scope) NumEntries() int { return len(s.entries) }
 
 // At returns the i'th scope entry for 0 <= i < NumEntries().
 func (s *Scope) At(i int) Object { return s.entries[i] }
 
 // NumChildren() returns the number of scopes nested in s.
-// If s == nil, the result is 0.
-func (s *Scope) NumChildren() int {
-	if s == nil {
-		return 0
-	}
-	return len(s.children)
-}
+func (s *Scope) NumChildren() int { return len(s.children) }
 
 // Child returns the i'th child scope for 0 <= i < NumChildren().
 func (s *Scope) Child(i int) *Scope { return s.children[i] }
 
-// Lookup returns the object in scope s with the given package
-// and name if such an object exists; otherwise the result is nil.
-// A nil scope acts like an empty scope, and parent scopes are ignored.
-//
-// If pkg != nil, both pkg.Path() and name are used to identify an
-// entry, per the Go rules for identifier equality. If pkg == nil,
-// only the name is used and the package path is ignored.
-func (s *Scope) Lookup(pkg *Package, name string) Object {
-	if s == nil {
-		return nil // empty scope
+// Lookup returns the object in scope s with the given name if such an
+// object exists; otherwise the result is nil.
+func (s *Scope) Lookup(name string) Object {
+	if s.objmap != nil {
+		return s.objmap[name]
 	}
-
-	// fast path: only the name must match
-	if pkg == nil {
-		for _, obj := range s.entries {
-			if obj.Name() == name {
-				return obj
-			}
-		}
-		return nil
-	}
-
-	// slow path: both pkg path and name must match
 	for _, obj := range s.entries {
-		if obj.sameId(pkg, name) {
+		if obj.Name() == name {
 			return obj
 		}
 	}
-
-	// not found
 	return nil
-
-	// TODO(gri) Optimize Lookup by also maintaining a map representation
-	//           for larger scopes.
 }
 
-// LookupParent follows the parent chain of scopes starting with s until it finds
-// a scope where Lookup(nil, name) returns a non-nil object, and then returns that
-// object. If no such scope exists, the result is nil.
+// LookupParent follows the parent chain of scopes starting with s until
+// it finds a scope where Lookup(name) returns a non-nil object, and then
+// returns that object. If no such scope exists, the result is nil.
 func (s *Scope) LookupParent(name string) Object {
-	for s != nil {
-		if obj := s.Lookup(nil, name); obj != nil {
+	for ; s != nil; s = s.parent {
+		if obj := s.Lookup(name); obj != nil {
 			return obj
 		}
-		s = s.parent
 	}
 	return nil
 }
@@ -135,19 +103,34 @@ func (s *Scope) LookupParent(name string) Object {
 // TODO(gri): Should Insert not be exported?
 
 // Insert attempts to insert an object obj into scope s.
-// If s already contains an object with the same package path
-// and name, Insert leaves s unchanged and returns that object.
-// Otherwise it inserts obj, sets the object's scope to s, and
-// returns nil. The object must not have the blank _ name.
-//
+// If s already contains an alternative object alt with
+// the same name, Insert leaves s unchanged and returns alt.
+// Otherwise it inserts obj, sets the object's scope to
+// s, and returns nil. The object name must not be blank _.
 func (s *Scope) Insert(obj Object) Object {
 	name := obj.Name()
 	assert(name != "_")
-	if alt := s.Lookup(obj.Pkg(), name); alt != nil {
+	if alt := s.Lookup(name); alt != nil {
 		return alt
 	}
+
+	// populate parallel objmap for larger scopes
+	// TODO(gri) what is the right threshold? should we only use a map?
+	if len(s.entries) == 32 {
+		m := make(map[string]Object)
+		for _, obj := range s.entries {
+			m[obj.Name()] = obj
+		}
+		s.objmap = m
+	}
+
+	// add object
 	s.entries = append(s.entries, obj)
+	if s.objmap != nil {
+		s.objmap[name] = obj
+	}
 	obj.setParent(s)
+
 	return nil
 }
 
@@ -159,7 +142,7 @@ func (s *Scope) WriteTo(w io.Writer, n int, recurse bool) {
 	const ind = ".  "
 	indn := strings.Repeat(ind, n)
 
-	if s.NumEntries() == 0 {
+	if len(s.entries) == 0 {
 		fmt.Fprintf(w, "%sscope %p {}\n", indn, s)
 		return
 	}
diff --git a/go/types/stmt.go b/go/types/stmt.go
index 5930a3c3..a55bb5bd 100644
--- a/go/types/stmt.go
+++ b/go/types/stmt.go
@@ -85,7 +85,7 @@ func (check *checker) stmt(s ast.Stmt) {
 			check.funcSig.labels = scope
 		}
 		label := s.Label
-		check.declare(scope, label, NewLabel(label.Pos(), label.Name))
+		check.declareObj(scope, label, NewLabel(label.Pos(), label.Name))
 		check.stmt(s.Stmt)
 
 	case *ast.ExprStmt:
@@ -267,7 +267,7 @@ func (check *checker) stmt(s ast.Stmt) {
 		if tag == nil {
 			// use fake true tag value and position it at the opening { of the switch
 			ident := &ast.Ident{NamePos: s.Body.Lbrace, Name: "true"}
-			check.recordObject(ident, Universe.Lookup(nil, "true"))
+			check.recordObject(ident, Universe.Lookup("true"))
 			tag = ident
 		}
 		check.expr(&x, tag)
@@ -416,7 +416,7 @@ func (check *checker) stmt(s ast.Stmt) {
 					typ = x.typ
 				}
 				obj := NewVar(lhs.Pos(), check.pkg, lhs.Name, typ)
-				check.declare(check.topScope, nil, obj)
+				check.declareObj(check.topScope, nil, obj)
 				check.recordImplicit(clause, obj)
 			}
 			check.stmtList(clause.Body)
@@ -552,7 +552,7 @@ func (check *checker) stmt(s ast.Stmt) {
 
 			// declare variables
 			for i, ident := range idents {
-				check.declare(check.topScope, ident, vars[i])
+				check.declareObj(check.topScope, ident, vars[i])
 			}
 		} else {
 			// ordinary assignment
diff --git a/go/types/types.go b/go/types/types.go
index 0a459d21..c68a7006 100644
--- a/go/types/types.go
+++ b/go/types/types.go
@@ -193,7 +193,7 @@ func (t *Tuple) At(i int) *Var { return t.vars[i] }
 
 // A Signature represents a (non-builtin) function type.
 type Signature struct {
-	scope      *Scope // function scope
+	scope      *Scope // function scope, always present
 	labels     *Scope // label scope, or nil (lazily allocated)
 	recv       *Var   // nil if not a method
 	params     *Tuple // (incoming) parameters from left to right; or nil
@@ -205,7 +205,9 @@ type Signature struct {
 // and results, either of which may be nil. If isVariadic is set, the function
 // is variadic, it must have at least one parameter, and the last parameter
 // must be of unnamed slice type.
-func NewSignature(recv *Var, params, results *Tuple, isVariadic bool) *Signature {
+func NewSignature(scope *Scope, recv *Var, params, results *Tuple, isVariadic bool) *Signature {
+	// TODO(gri) Should we rely on the correct (non-nil) incoming scope
+	//           or should this function allocate and populate a scope?
 	if isVariadic {
 		n := params.Len()
 		if n == 0 {
@@ -215,7 +217,7 @@ func NewSignature(recv *Var, params, results *Tuple, isVariadic bool) *Signature
 			panic("types.NewSignature: variadic parameter must be of unnamed slice type")
 		}
 	}
-	return &Signature{nil, nil, recv, params, results, isVariadic}
+	return &Signature{scope, nil, recv, params, results, isVariadic}
 }
 
 // Recv returns the receiver of signature s, or nil.
diff --git a/go/types/types_test.go b/go/types/types_test.go
index 869d10e3..a3dd631f 100644
--- a/go/types/types_test.go
+++ b/go/types/types_test.go
@@ -110,7 +110,7 @@ func TestTypes(t *testing.T) {
 			t.Errorf("%s: %s", src, err)
 			continue
 		}
-		typ := pkg.scope.Lookup(nil, "T").Type().Underlying()
+		typ := pkg.scope.Lookup("T").Type().Underlying()
 		str := typeString(typ)
 		if str != test.str {
 			t.Errorf("%s: got %s, want %s", test.src, str, test.str)
diff --git a/go/types/typexpr.go b/go/types/typexpr.go
index e649be18..91a93b13 100644
--- a/go/types/typexpr.go
+++ b/go/types/typexpr.go
@@ -360,7 +360,7 @@ func (check *checker) collectParams(scope *Scope, list *ast.FieldList, variadicO
 			// named parameter
 			for _, name := range field.Names {
 				par := NewVar(name.Pos(), check.pkg, name.Name, typ)
-				check.declare(scope, name, par)
+				check.declareObj(scope, name, par)
 				params = append(params, par)
 			}
 		} else {
@@ -385,7 +385,8 @@ func (check *checker) collectMethods(recv Type, list *ast.FieldList, cycleOk boo
 		return nil
 	}
 
-	scope := NewScope(nil)
+	var mset objset
+
 	for _, f := range list.List {
 		// TODO(gri) Consider calling funcType here.
 		typ := check.typ(f.Type, nil, cycleOk)
@@ -402,7 +403,7 @@ func (check *checker) collectMethods(recv Type, list *ast.FieldList, cycleOk boo
 			sig.recv = NewVar(token.NoPos, check.pkg, "", recv)
 			for _, name := range f.Names {
 				m := NewFunc(name.Pos(), check.pkg, name.Name, sig)
-				check.declare(scope, name, m)
+				check.declareFld(&mset, name, m)
 				methods = append(methods, m)
 			}
 		} else {
@@ -417,7 +418,7 @@ func (check *checker) collectMethods(recv Type, list *ast.FieldList, cycleOk boo
 				check.errorf(f.Type.Pos(), "reference to incomplete type %s - unimplemented", f.Type)
 			case *Interface:
 				for _, m := range t.methods {
-					check.declare(scope, nil, m)
+					check.declareFld(&mset, nil, m)
 					methods = append(methods, m)
 				}
 			default:
@@ -448,7 +449,7 @@ func (check *checker) collectFields(list *ast.FieldList, cycleOk bool) (fields [
 		return
 	}
 
-	scope := NewScope(nil)
+	var fset objset
 
 	var typ Type   // current field typ
 	var tag string // current field tag
@@ -461,7 +462,7 @@ func (check *checker) collectFields(list *ast.FieldList, cycleOk bool) (fields [
 		}
 
 		fld := NewFieldVar(pos, check.pkg, name, typ, anonymous)
-		check.declare(scope, ident, fld)
+		check.declareFld(&fset, ident, fld)
 		fields = append(fields, fld)
 	}
 
diff --git a/go/types/universe.go b/go/types/universe.go
index 75231c79..114f0744 100644
--- a/go/types/universe.go
+++ b/go/types/universe.go
@@ -116,7 +116,7 @@ func init() {
 		def(NewFunc(token.NoPos, nil, f.name, f))
 	}
 
-	universeIota = Universe.Lookup(nil, "iota").(*Const)
+	universeIota = Universe.Lookup("iota").(*Const)
 }
 
 // Objects with names containing blanks are internal and not entered into
diff --git a/importer/pkginfo.go b/importer/pkginfo.go
index 07cb71ff..7392fe90 100644
--- a/importer/pkginfo.go
+++ b/importer/pkginfo.go
@@ -116,7 +116,7 @@ func (info *PackageInfo) IsType(e ast.Expr) bool {
 func (info *PackageInfo) IsPackageRef(sel *ast.SelectorExpr) types.Object {
 	if id, ok := sel.X.(*ast.Ident); ok {
 		if pkg, ok := info.ObjectOf(id).(*types.Package); ok {
-			return pkg.Scope().Lookup(nil, sel.Sel.Name)
+			return pkg.Scope().Lookup(sel.Sel.Name)
 		}
 	}
 	return nil
@@ -245,5 +245,5 @@ func (info *PackageInfo) BuiltinCallSignature(e *ast.CallExpr) *types.Signature
 		panic("unknown builtin: " + builtin)
 	}
 
-	return types.NewSignature(nil, types.NewTuple(params...), nil, isVariadic)
+	return types.NewSignature(nil, nil, types.NewTuple(params...), nil, isVariadic)
 }
diff --git a/ssa/builder.go b/ssa/builder.go
index 4cf400ad..50e1cbc3 100644
--- a/ssa/builder.go
+++ b/ssa/builder.go
@@ -1716,7 +1716,7 @@ func (b *builder) rangeIndexed(fn *Function, x Value, tv types.Type) (k, v Value
 	} else {
 		// length = len(x).
 		var c Call
-		c.Call.Func = fn.Prog.builtins[types.Universe.Lookup(nil, "len")]
+		c.Call.Func = fn.Prog.builtins[types.Universe.Lookup("len")]
 		c.Call.Args = []Value{x}
 		c.setType(tInt)
 		length = fn.emit(&c)
diff --git a/ssa/interp/reflect.go b/ssa/interp/reflect.go
index 3e3dd9c3..43632751 100644
--- a/ssa/interp/reflect.go
+++ b/ssa/interp/reflect.go
@@ -394,7 +394,7 @@ func newMethod(pkg *ssa.Package, recvType types.Type, name string) *ssa.Function
 	// that is needed is the "pointerness" of Recv.Type, and for
 	// now, we'll set it to always be false since we're only
 	// concerned with rtype.  Encapsulate this better.
-	sig := types.NewSignature(types.NewVar(token.NoPos, nil, "recv", recvType), nil, nil, false)
+	sig := types.NewSignature(nil, types.NewVar(token.NoPos, nil, "recv", recvType), nil, nil, false)
 	fn := ssa.NewFunction(name, sig, "fake reflect method")
 	fn.Pkg = pkg
 	fn.Prog = pkg.Prog
diff --git a/ssa/promote.go b/ssa/promote.go
index 822e68c8..82323380 100644
--- a/ssa/promote.go
+++ b/ssa/promote.go
@@ -140,7 +140,7 @@ func makeMethod(prog *Program, typ types.Type, obj *types.Method) *Function {
 //
 func promotionWrapper(prog *Program, typ types.Type, obj *types.Method) *Function {
 	old := obj.Func.Type().(*types.Signature)
-	sig := types.NewSignature(types.NewVar(token.NoPos, nil, "recv", typ), old.Params(), old.Results(), old.IsVariadic())
+	sig := types.NewSignature(nil, types.NewVar(token.NoPos, nil, "recv", typ), old.Params(), old.Results(), old.IsVariadic())
 
 	// TODO(adonovan): include implicit field path in description.
 	description := fmt.Sprintf("promotion wrapper for (%s).%s", old.Recv(), obj.Func.Name())
@@ -310,7 +310,7 @@ func boundMethodWrapper(meth *Function) *Function {
 		s := meth.Signature
 		fn = &Function{
 			name:      "bound$" + meth.String(),
-			Signature: types.NewSignature(nil, s.Params(), s.Results(), s.IsVariadic()), // drop recv
+			Signature: types.NewSignature(nil, nil, s.Params(), s.Results(), s.IsVariadic()), // drop recv
 			Synthetic: "bound method wrapper for " + meth.String(),
 			Prog:      prog,
 			pos:       meth.Pos(),
@@ -359,7 +359,7 @@ func indirectionWrapper(meth *Function) *Function {
 		// TODO(adonovan): is there a *types.Func for this method?
 		fn = &Function{
 			name:      meth.Name(),
-			Signature: types.NewSignature(recv, s.Params(), s.Results(), s.IsVariadic()),
+			Signature: types.NewSignature(nil, recv, s.Params(), s.Results(), s.IsVariadic()),
 			Prog:      prog,
 			Synthetic: "receiver indirection wrapper for " + meth.String(),
 			pos:       meth.Pos(),