reflect: add PtrTo, add Value.Addr (old Addr is now UnsafeAddr)

This change makes it possible to take the address of a
struct field or slice element in order to call a method that
requires a pointer receiver.

Existing code that uses the Value.Addr method will have
to change (as gob does in this CL) to call UnsafeAddr instead.

R=r, rog
CC=golang-dev
https://golang.org/cl/4239052

6 files changed, 285 insertions, 52 deletions

diff --git a/src/pkg/gob/decode.go b/src/pkg/gob/decode.go
index ad03d176b8..8f599e1004 100644
--- a/src/pkg/gob/decode.go
+++ b/src/pkg/gob/decode.go
@@ -518,7 +518,7 @@ func (dec *Decoder) decodeArray(atyp *reflect.ArrayType, state *decodeState, p u
 
 func decodeIntoValue(state *decodeState, op decOp, indir int, v reflect.Value, ovfl os.ErrorString) reflect.Value {
 	instr := &decInstr{op, 0, indir, 0, ovfl}
-	up := unsafe.Pointer(v.Addr())
+	up := unsafe.Pointer(v.UnsafeAddr())
 	if indir > 1 {
 		up = decIndirect(up, indir)
 	}
@@ -1052,9 +1052,9 @@ func (dec *Decoder) decodeValue(wireId typeId, val reflect.Value) (err os.Error)
 			name := base.Name()
 			return os.ErrorString("gob: type mismatch: no fields matched compiling decoder for " + name)
 		}
-		return dec.decodeStruct(engine, ut, uintptr(val.Addr()), indir)
+		return dec.decodeStruct(engine, ut, uintptr(val.UnsafeAddr()), indir)
 	}
-	return dec.decodeSingle(engine, ut, uintptr(val.Addr()))
+	return dec.decodeSingle(engine, ut, uintptr(val.UnsafeAddr()))
 }
 
 func (dec *Decoder) decodeIgnoredValue(wireId typeId) os.Error {
diff --git a/src/pkg/gob/encode.go b/src/pkg/gob/encode.go
index 5f4fc6f34b..e92db74ffd 100644
--- a/src/pkg/gob/encode.go
+++ b/src/pkg/gob/encode.go
@@ -356,7 +356,7 @@ func encodeReflectValue(state *encoderState, v reflect.Value, op encOp, indir in
 	if v == nil {
 		errorf("gob: encodeReflectValue: nil element")
 	}
-	op(nil, state, unsafe.Pointer(v.Addr()))
+	op(nil, state, unsafe.Pointer(v.UnsafeAddr()))
 }
 
 func (enc *Encoder) encodeMap(b *bytes.Buffer, mv *reflect.MapValue, keyOp, elemOp encOp, keyIndir, elemIndir int) {
@@ -575,9 +575,9 @@ func (enc *Encoder) encode(b *bytes.Buffer, value reflect.Value, ut *userTypeInf
 	}
 	engine := enc.lockAndGetEncEngine(ut.base)
 	if value.Type().Kind() == reflect.Struct {
-		enc.encodeStruct(b, engine, value.Addr())
+		enc.encodeStruct(b, engine, value.UnsafeAddr())
 	} else {
-		enc.encodeSingle(b, engine, value.Addr())
+		enc.encodeSingle(b, engine, value.UnsafeAddr())
 	}
 	return nil
 }
diff --git a/src/pkg/reflect/all_test.go b/src/pkg/reflect/all_test.go
index 3675be6f1c..7a97ea1737 100644
--- a/src/pkg/reflect/all_test.go
+++ b/src/pkg/reflect/all_test.go
@@ -1093,6 +1093,18 @@ func TestMethod(t *testing.T) {
 		t.Errorf("Value Method returned %d; want 250", i)
 	}
 
+	// Curried method of pointer.
+	i = NewValue(&p).Method(0).Call([]Value{NewValue(10)})[0].(*IntValue).Get()
+	if i != 250 {
+		t.Errorf("Value Method returned %d; want 250", i)
+	}
+
+	// Curried method of pointer to value.
+	i = NewValue(p).Addr().Method(0).Call([]Value{NewValue(10)})[0].(*IntValue).Get()
+	if i != 250 {
+		t.Errorf("Value Method returned %d; want 250", i)
+	}
+
 	// Curried method of interface value.
 	// Have to wrap interface value in a struct to get at it.
 	// Passing it to NewValue directly would
@@ -1390,3 +1402,66 @@ func TestEmbeddedMethods(t *testing.T) {
 		t.Errorf("f(o) = %d, want 2", v)
 	}
 }
+
+func TestPtrTo(t *testing.T) {
+	var i int
+
+	typ := Typeof(i)
+	for i = 0; i < 100; i++ {
+		typ = PtrTo(typ)
+	}
+	for i = 0; i < 100; i++ {
+		typ = typ.(*PtrType).Elem()
+	}
+	if typ != Typeof(i) {
+		t.Errorf("after 100 PtrTo and Elem, have %s, want %s", typ, Typeof(i))
+	}
+}
+
+func TestAddr(t *testing.T) {
+	var p struct {
+		X, Y int
+	}
+
+	v := NewValue(&p)
+	v = v.(*PtrValue).Elem()
+	v = v.Addr()
+	v = v.(*PtrValue).Elem()
+	v = v.(*StructValue).Field(0)
+	v.(*IntValue).Set(2)
+	if p.X != 2 {
+		t.Errorf("Addr.Elem.Set failed to set value")
+	}
+
+	// Again but take address of the NewValue value.
+	// Exercises generation of PtrTypes not present in the binary.
+	v = NewValue(&p)
+	v = v.Addr()
+	v = v.(*PtrValue).Elem()
+	v = v.(*PtrValue).Elem()
+	v = v.Addr()
+	v = v.(*PtrValue).Elem()
+	v = v.(*StructValue).Field(0)
+	v.(*IntValue).Set(3)
+	if p.X != 3 {
+		t.Errorf("Addr.Elem.Set failed to set value")
+	}
+
+	// Starting without pointer we should get changed value
+	// in interface.
+	v = NewValue(p)
+	v0 := v
+	v = v.Addr()
+	v = v.(*PtrValue).Elem()
+	v = v.(*StructValue).Field(0)
+	v.(*IntValue).Set(4)
+	if p.X != 3 { // should be unchanged from last time
+		t.Errorf("somehow value Set changed original p")
+	}
+	p = v0.Interface().(struct {
+		X, Y int
+	})
+	if p.X != 4 {
+		t.Errorf("Addr.Elem.Set valued to set value in top value")
+	}
+}
diff --git a/src/pkg/reflect/deepequal.go b/src/pkg/reflect/deepequal.go
index a50925e51e..c9beec5066 100644
--- a/src/pkg/reflect/deepequal.go
+++ b/src/pkg/reflect/deepequal.go
@@ -31,8 +31,8 @@ func deepValueEqual(v1, v2 Value, visited map[uintptr]*visit, depth int) bool {
 
 	// if depth > 10 { panic("deepValueEqual") }	// for debugging
 
-	addr1 := v1.Addr()
-	addr2 := v2.Addr()
+	addr1 := v1.UnsafeAddr()
+	addr2 := v2.UnsafeAddr()
 	if addr1 > addr2 {
 		// Canonicalize order to reduce number of entries in visited.
 		addr1, addr2 = addr2, addr1
diff --git a/src/pkg/reflect/type.go b/src/pkg/reflect/type.go
index efe0238eaa..2cc1f576aa 100644
--- a/src/pkg/reflect/type.go
+++ b/src/pkg/reflect/type.go
@@ -18,6 +18,7 @@ package reflect
 import (
 	"runtime"
 	"strconv"
+	"sync"
 	"unsafe"
 )
 
@@ -251,6 +252,8 @@ type Type interface {
 
 	// NumMethods returns the number of methods in the type's method set.
 	NumMethod() int
+
+	common() *commonType
 	uncommon() *uncommonType
 }
 
@@ -361,6 +364,8 @@ func (t *commonType) FieldAlign() int { return int(t.fieldAlign) }
 
 func (t *commonType) Kind() Kind { return Kind(t.kind & kindMask) }
 
+func (t *commonType) common() *commonType { return t }
+
 func (t *uncommonType) Method(i int) (m Method) {
 	if t == nil || i < 0 || i >= len(t.methods) {
 		return
@@ -374,7 +379,7 @@ func (t *uncommonType) Method(i int) (m Method) {
 	}
 	m.Type = toType(*p.typ).(*FuncType)
 	fn := p.tfn
-	m.Func = &FuncValue{value: value{m.Type, addr(&fn), true}}
+	m.Func = &FuncValue{value: value{m.Type, addr(&fn), canSet}}
 	return
 }
 
@@ -689,3 +694,84 @@ type ArrayOrSliceType interface {
 
 // Typeof returns the reflection Type of the value in the interface{}.
 func Typeof(i interface{}) Type { return toType(unsafe.Typeof(i)) }
+
+// ptrMap is the cache for PtrTo.
+var ptrMap struct {
+	sync.RWMutex
+	m map[Type]*PtrType
+}
+
+// runtimePtrType is the runtime layout for a *PtrType.
+// The memory immediately before the *PtrType is always
+// the canonical runtime.Type to be used for a *runtime.Type
+// describing this PtrType.
+type runtimePtrType struct {
+	runtime.Type
+	runtime.PtrType
+}
+
+// PtrTo returns the pointer type with element t.
+// For example, if t represents type Foo, PtrTo(t) represents *Foo.
+func PtrTo(t Type) *PtrType {
+	// If t records its pointer-to type, use it.
+	ct := t.common()
+	if p := ct.ptrToThis; p != nil {
+		return toType(*p).(*PtrType)
+	}
+
+	// Otherwise, synthesize one.
+	// This only happens for pointers with no methods.
+	// We keep the mapping in a map on the side, because
+	// this operation is rare and a separate map lets us keep
+	// the type structures in read-only memory.
+	ptrMap.RLock()
+	if m := ptrMap.m; m != nil {
+		if p := m[t]; p != nil {
+			ptrMap.RUnlock()
+			return p
+		}
+	}
+	ptrMap.RUnlock()
+	ptrMap.Lock()
+	if ptrMap.m == nil {
+		ptrMap.m = make(map[Type]*PtrType)
+	}
+	p := ptrMap.m[t]
+	if p != nil {
+		// some other goroutine won the race and created it
+		ptrMap.Unlock()
+		return p
+	}
+
+	// runtime.Type value is always right before type structure.
+	// 2*ptrSize is size of interface header
+	rt := (*runtime.Type)(unsafe.Pointer(uintptr(unsafe.Pointer(ct)) - uintptr(unsafe.Sizeof(runtime.Type(nil)))))
+
+	rp := new(runtimePtrType)
+	rp.Type = &rp.PtrType
+
+	// initialize rp.PtrType using *byte's PtrType as a prototype.
+	// have to do assignment as PtrType, not runtime.PtrType,
+	// in order to write to unexported fields.
+	p = (*PtrType)(unsafe.Pointer(&rp.PtrType))
+	bp := (*PtrType)(unsafe.Pointer(unsafe.Typeof((*byte)(nil)).(*runtime.PtrType)))
+	*p = *bp
+
+	s := "*" + *ct.string
+	p.string = &s
+
+	// For the type structures linked into the binary, the
+	// compiler provides a good hash of the string.
+	// Create a good hash for the new string by using
+	// the FNV-1 hash's mixing function to combine the
+	// old hash and the new "*".
+	p.hash = ct.hash*16777619 ^ '*'
+
+	p.uncommonType = nil
+	p.ptrToThis = nil
+	p.elem = rt
+
+	ptrMap.m[t] = (*PtrType)(unsafe.Pointer(&rp.PtrType))
+	ptrMap.Unlock()
+	return p
+}
diff --git a/src/pkg/reflect/value.go b/src/pkg/reflect/value.go
index 4d7d872373..0b70b17f8f 100644
--- a/src/pkg/reflect/value.go
+++ b/src/pkg/reflect/value.go
@@ -11,7 +11,7 @@ import (
 )
 
 const ptrSize = uintptr(unsafe.Sizeof((*byte)(nil)))
-const cannotSet = "cannot set value obtained via unexported struct field"
+const cannotSet = "cannot set value obtained from unexported struct field"
 
 type addr unsafe.Pointer
 
@@ -51,20 +51,32 @@ type Value interface {
 	// Interface returns the value as an interface{}.
 	Interface() interface{}
 
-	// CanSet returns whether the value can be changed.
+	// CanSet returns true if the value can be changed.
 	// Values obtained by the use of non-exported struct fields
 	// can be used in Get but not Set.
-	// If CanSet() returns false, calling the type-specific Set
-	// will cause a crash.
+	// If CanSet returns false, calling the type-specific Set will panic.
 	CanSet() bool
 
 	// SetValue assigns v to the value; v must have the same type as the value.
 	SetValue(v Value)
 
-	// Addr returns a pointer to the underlying data.
-	// It is for advanced clients that also
-	// import the "unsafe" package.
-	Addr() uintptr
+	// CanAddr returns true if the value's address can be obtained with Addr.
+	// Such values are called addressable.  A value is addressable if it is
+	// an element of a slice, an element of an addressable array,
+	// a field of an addressable struct, the result of dereferencing a pointer,
+	// or the result of a call to NewValue, MakeChan, MakeMap, or MakeZero.
+	// If CanAddr returns false, calling Addr will panic.
+	CanAddr() bool
+
+	// Addr returns the address of the value.
+	// If the value is not addressable, Addr panics.
+	// Addr is typically used to obtain a pointer to a struct field or slice element
+	// in order to call a method that requires a pointer receiver.
+	Addr() *PtrValue
+
+	// UnsafeAddr returns a pointer to the underlying data.
+	// It is for advanced clients that also import the "unsafe" package.
+	UnsafeAddr() uintptr
 
 	// Method returns a FuncValue corresponding to the value's i'th method.
 	// The arguments to a Call on the returned FuncValue
@@ -75,19 +87,42 @@ type Value interface {
 	getAddr() addr
 }
 
+// flags for value
+const (
+	canSet   uint32 = 1 << iota // can set value (write to *v.addr)
+	canAddr                     // can take address of value
+	canStore                    // can store through value (write to **v.addr)
+)
+
 // value is the common implementation of most values.
 // It is embedded in other, public struct types, but always
 // with a unique tag like "uint" or "float" so that the client cannot
 // convert from, say, *UintValue to *FloatValue.
 type value struct {
-	typ    Type
-	addr   addr
-	canSet bool
+	typ  Type
+	addr addr
+	flag uint32
 }
 
 func (v *value) Type() Type { return v.typ }
 
-func (v *value) Addr() uintptr { return uintptr(v.addr) }
+func (v *value) Addr() *PtrValue {
+	if !v.CanAddr() {
+		panic("reflect: cannot take address of value")
+	}
+	a := v.addr
+	flag := canSet
+	if v.CanSet() {
+		flag |= canStore
+	}
+	// We could safely set canAddr here too -
+	// the caller would get the address of a -
+	// but it doesn't match the Go model.
+	// The language doesn't let you say &&v.
+	return newValue(PtrTo(v.typ), addr(&a), flag).(*PtrValue)
+}
+
+func (v *value) UnsafeAddr() uintptr { return uintptr(v.addr) }
 
 func (v *value) getAddr() addr { return v.addr }
 
@@ -109,7 +144,10 @@ func (v *value) Interface() interface{} {
 	return unsafe.Unreflect(v.typ, unsafe.Pointer(v.addr))
 }
 
-func (v *value) CanSet() bool { return v.canSet }
+func (v *value) CanSet() bool { return v.flag&canSet != 0 }
+
+func (v *value) CanAddr() bool { return v.flag&canAddr != 0 }
+
 
 /*
  * basic types
@@ -125,7 +163,7 @@ func (v *BoolValue) Get() bool { return *(*bool)(v.addr) }
 
 // Set sets v to the value x.
 func (v *BoolValue) Set(x bool) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	*(*bool)(v.addr) = x
@@ -152,7 +190,7 @@ func (v *FloatValue) Get() float64 {
 
 // Set sets v to the value x.
 func (v *FloatValue) Set(x float64) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	switch v.typ.Kind() {
@@ -197,7 +235,7 @@ func (v *ComplexValue) Get() complex128 {
 
 // Set sets v to the value x.
 func (v *ComplexValue) Set(x complex128) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	switch v.typ.Kind() {
@@ -237,7 +275,7 @@ func (v *IntValue) Get() int64 {
 
 // Set sets v to the value x.
 func (v *IntValue) Set(x int64) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	switch v.typ.Kind() {
@@ -282,7 +320,7 @@ func (v *StringValue) Get() string { return *(*string)(v.addr) }
 
 // Set sets v to the value x.
 func (v *StringValue) Set(x string) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	*(*string)(v.addr) = x
@@ -317,7 +355,7 @@ func (v *UintValue) Get() uint64 {
 
 // Set sets v to the value x.
 func (v *UintValue) Set(x uint64) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	switch v.typ.Kind() {
@@ -361,7 +399,7 @@ func (v *UnsafePointerValue) Get() uintptr { return uintptr(*(*unsafe.Pointer)(v
 
 // Set sets v to the value x.
 func (v *UnsafePointerValue) Set(x unsafe.Pointer) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	*(*unsafe.Pointer)(v.addr) = x
@@ -473,7 +511,7 @@ func (v *ArrayValue) addr() addr { return v.value.addr }
 // Set assigns x to v.
 // The new value x must have the same type as v.
 func (v *ArrayValue) Set(x *ArrayValue) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	typesMustMatch(v.typ, x.typ)
@@ -491,7 +529,7 @@ func (v *ArrayValue) Elem(i int) Value {
 		panic("array index out of bounds")
 	}
 	p := addr(uintptr(v.addr()) + uintptr(i)*typ.Size())
-	return newValue(typ, p, v.canSet)
+	return newValue(typ, p, v.flag)
 }
 
 /*
@@ -537,7 +575,7 @@ func (v *SliceValue) SetLen(n int) {
 // Set assigns x to v.
 // The new value x must have the same type as v.
 func (v *SliceValue) Set(x *SliceValue) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	typesMustMatch(v.typ, x.typ)
@@ -566,7 +604,14 @@ func (v *SliceValue) Slice(beg, end int) *SliceValue {
 	s.Data = uintptr(v.addr()) + uintptr(beg)*typ.Elem().Size()
 	s.Len = end - beg
 	s.Cap = cap - beg
-	return newValue(typ, addr(s), v.canSet).(*SliceValue)
+
+	// Like the result of Addr, we treat Slice as an
+	// unaddressable temporary, so don't set canAddr.
+	flag := canSet
+	if v.flag&canStore != 0 {
+		flag |= canStore
+	}
+	return newValue(typ, addr(s), flag).(*SliceValue)
 }
 
 // Elem returns the i'th element of v.
@@ -577,7 +622,11 @@ func (v *SliceValue) Elem(i int) Value {
 		panic("reflect: slice index out of range")
 	}
 	p := addr(uintptr(v.addr()) + uintptr(i)*typ.Size())
-	return newValue(typ, p, v.canSet)
+	flag := canAddr
+	if v.flag&canStore != 0 {
+		flag |= canSet | canStore
+	}
+	return newValue(typ, p, flag)
 }
 
 // MakeSlice creates a new zero-initialized slice value
@@ -588,7 +637,7 @@ func MakeSlice(typ *SliceType, len, cap int) *SliceValue {
 		Len:  len,
 		Cap:  cap,
 	}
-	return newValue(typ, addr(s), true).(*SliceValue)
+	return newValue(typ, addr(s), canAddr|canSet|canStore).(*SliceValue)
 }
 
 /*
@@ -606,7 +655,7 @@ func (v *ChanValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }
 // Set assigns x to v.
 // The new value x must have the same type as v.
 func (v *ChanValue) Set(x *ChanValue) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	typesMustMatch(v.typ, x.typ)
@@ -733,7 +782,7 @@ func (v *FuncValue) Get() uintptr { return *(*uintptr)(v.addr) }
 // Set assigns x to v.
 // The new value x must have the same type as v.
 func (v *FuncValue) Set(x *FuncValue) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	typesMustMatch(v.typ, x.typ)
@@ -754,7 +803,7 @@ func (v *value) Method(i int) *FuncValue {
 	}
 	p := &t.methods[i]
 	fn := p.tfn
-	fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), true}, first: v, isInterface: false}
+	fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), 0}, first: v, isInterface: false}
 	return fv
 }
 
@@ -765,6 +814,17 @@ type tiny struct {
 	b byte
 }
 
+// Interface returns the fv as an interface value.
+// If fv is a method obtained by invoking Value.Method
+// (as opposed to Type.Method), Interface cannot return an
+// interface value, so it panics.
+func (fv *FuncValue) Interface() interface{} {
+	if fv.first != nil {
+		panic("FuncValue: cannot create interface value for method with bound receiver")
+	}
+	return fv.value.Interface()
+}
+
 // Call calls the function fv with input parameters in.
 // It returns the function's output parameters as Values.
 func (fv *FuncValue) Call(in []Value) []Value {
@@ -902,7 +962,7 @@ func (v *InterfaceValue) Set(x Value) {
 	if x != nil {
 		i = x.Interface()
 	}
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	// Two different representations; see comment in Get.
@@ -933,11 +993,11 @@ func (v *InterfaceValue) Method(i int) *FuncValue {
 
 	// Interface is two words: itable, data.
 	tab := *(**runtime.Itable)(v.addr)
-	data := &value{Typeof((*byte)(nil)), addr(uintptr(v.addr) + ptrSize), true}
+	data := &value{Typeof((*byte)(nil)), addr(uintptr(v.addr) + ptrSize), 0}
 
 	// Function pointer is at p.perm in the table.
 	fn := tab.Fn[i]
-	fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), true}, first: data, isInterface: true}
+	fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), 0}, first: data, isInterface: true}
 	return fv
 }
 
@@ -956,7 +1016,7 @@ func (v *MapValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }
 // Set assigns x to v.
 // The new value x must have the same type as v.
 func (v *MapValue) Set(x *MapValue) {
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	if x == nil {
@@ -1075,15 +1135,18 @@ func (v *PtrValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }
 func (v *PtrValue) Get() uintptr { return *(*uintptr)(v.addr) }
 
 // Set assigns x to v.
-// The new value x must have the same type as v.
+// The new value x must have the same type as v, and x.Elem().CanSet() must be true.
 func (v *PtrValue) Set(x *PtrValue) {
 	if x == nil {
 		*(**uintptr)(v.addr) = nil
 		return
 	}
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
+	if x.flag&canStore == 0 {
+		panic("cannot copy pointer obtained from unexported struct field")
+	}
 	typesMustMatch(v.typ, x.typ)
 	// TODO: This will have to move into the runtime
 	// once the new gc goes in
@@ -1112,7 +1175,7 @@ func (v *PtrValue) PointTo(x Value) {
 	typesMustMatch(v.typ.(*PtrType).Elem(), x.Type())
 	// TODO: This will have to move into the runtime
 	// once the new gc goes in.
-	*(*uintptr)(v.addr) = x.Addr()
+	*(*uintptr)(v.addr) = x.UnsafeAddr()
 }
 
 // Elem returns the value that v points to.
@@ -1121,7 +1184,11 @@ func (v *PtrValue) Elem() Value {
 	if v.IsNil() {
 		return nil
 	}
-	return newValue(v.typ.(*PtrType).Elem(), *(*addr)(v.addr), v.canSet)
+	flag := canAddr
+	if v.flag&canStore != 0 {
+		flag |= canSet | canStore
+	}
+	return newValue(v.typ.(*PtrType).Elem(), *(*addr)(v.addr), flag)
 }
 
 // Indirect returns the value that v points to.
@@ -1148,7 +1215,7 @@ type StructValue struct {
 func (v *StructValue) Set(x *StructValue) {
 	// TODO: This will have to move into the runtime
 	// once the gc goes in.
-	if !v.canSet {
+	if !v.CanSet() {
 		panic(cannotSet)
 	}
 	typesMustMatch(v.typ, x.typ)
@@ -1165,7 +1232,12 @@ func (v *StructValue) Field(i int) Value {
 		return nil
 	}
 	f := t.Field(i)
-	return newValue(f.Type, addr(uintptr(v.addr)+f.Offset), v.canSet && f.PkgPath == "")
+	flag := v.flag
+	if f.PkgPath != "" {
+		// unexported field
+		flag &^= canSet | canStore
+	}
+	return newValue(f.Type, addr(uintptr(v.addr)+f.Offset), flag)
 }
 
 // FieldByIndex returns the nested field corresponding to index.
@@ -1221,11 +1293,11 @@ func NewValue(i interface{}) Value {
 		return nil
 	}
 	t, a := unsafe.Reflect(i)
-	return newValue(toType(t), addr(a), true)
+	return newValue(toType(t), addr(a), canSet|canAddr|canStore)
 }
 
-func newValue(typ Type, addr addr, canSet bool) Value {
-	v := value{typ, addr, canSet}
+func newValue(typ Type, addr addr, flag uint32) Value {
+	v := value{typ, addr, flag}
 	switch typ.(type) {
 	case *ArrayType:
 		return &ArrayValue{v}
@@ -1266,5 +1338,5 @@ func MakeZero(typ Type) Value {
 	if typ == nil {
 		return nil
 	}
-	return newValue(typ, addr(unsafe.New(typ)), true)
+	return newValue(typ, addr(unsafe.New(typ)), canSet|canAddr|canStore)
 }