reflect: disallow Interface method on Value obtained via unexported name

Had been allowing it for use by fmt, but it is too hard to lock down.
Fix other packages not to depend on it.

R=r, r
CC=golang-dev
https://golang.org/cl/5266054

src/pkg/reflect/all_test.go

@@ -853,13 +853,13 @@ func TestIsNil(t *testing.T) {
 
 func TestInterfaceExtraction(t *testing.T) {
 	var s struct {
-		w io.Writer
+		W io.Writer
 	}
 
-	s.w = os.Stdout
+	s.W = os.Stdout
 	v := Indirect(ValueOf(&s)).Field(0).Interface()
-	if v != s.w.(interface{}) {
-		t.Error("Interface() on interface: ", v, s.w)
+	if v != s.W.(interface{}) {
+		t.Error("Interface() on interface: ", v, s.W)
 	}
 }
 
@@ -1190,18 +1190,18 @@ type D2 struct {
 }
 
 type S0 struct {
-	a, b, c int
+	A, B, C int
 	D1
 	D2
 }
 
 type S1 struct {
-	b int
+	B int
 	S0
 }
 
 type S2 struct {
-	a int
+	A int
 	*S1
 }
 
@@ -1216,36 +1216,36 @@ type S1y struct {
 type S3 struct {
 	S1x
 	S2
-	d, e int
+	D, E int
 	*S1y
 }
 
 type S4 struct {
 	*S4
-	a int
+	A int
 }
 
 var fieldTests = []FTest{
 	{struct{}{}, "", nil, 0},
-	{struct{}{}, "foo", nil, 0},
-	{S0{a: 'a'}, "a", []int{0}, 'a'},
-	{S0{}, "d", nil, 0},
-	{S1{S0: S0{a: 'a'}}, "a", []int{1, 0}, 'a'},
-	{S1{b: 'b'}, "b", []int{0}, 'b'},
+	{struct{}{}, "Foo", nil, 0},
+	{S0{A: 'a'}, "A", []int{0}, 'a'},
+	{S0{}, "D", nil, 0},
+	{S1{S0: S0{A: 'a'}}, "A", []int{1, 0}, 'a'},
+	{S1{B: 'b'}, "B", []int{0}, 'b'},
 	{S1{}, "S0", []int{1}, 0},
-	{S1{S0: S0{c: 'c'}}, "c", []int{1, 2}, 'c'},
-	{S2{a: 'a'}, "a", []int{0}, 'a'},
+	{S1{S0: S0{C: 'c'}}, "C", []int{1, 2}, 'c'},
+	{S2{A: 'a'}, "A", []int{0}, 'a'},
 	{S2{}, "S1", []int{1}, 0},
-	{S2{S1: &S1{b: 'b'}}, "b", []int{1, 0}, 'b'},
-	{S2{S1: &S1{S0: S0{c: 'c'}}}, "c", []int{1, 1, 2}, 'c'},
-	{S2{}, "d", nil, 0},
+	{S2{S1: &S1{B: 'b'}}, "B", []int{1, 0}, 'b'},
+	{S2{S1: &S1{S0: S0{C: 'c'}}}, "C", []int{1, 1, 2}, 'c'},
+	{S2{}, "D", nil, 0},
 	{S3{}, "S1", nil, 0},
-	{S3{S2: S2{a: 'a'}}, "a", []int{1, 0}, 'a'},
-	{S3{}, "b", nil, 0},
-	{S3{d: 'd'}, "d", []int{2}, 0},
-	{S3{e: 'e'}, "e", []int{3}, 'e'},
-	{S4{a: 'a'}, "a", []int{1}, 'a'},
-	{S4{}, "b", nil, 0},
+	{S3{S2: S2{A: 'a'}}, "A", []int{1, 0}, 'a'},
+	{S3{}, "B", nil, 0},
+	{S3{D: 'd'}, "D", []int{2}, 0},
+	{S3{E: 'e'}, "E", []int{3}, 'e'},
+	{S4{A: 'a'}, "A", []int{1}, 'a'},
+	{S4{}, "B", nil, 0},
 }
 
 func TestFieldByIndex(t *testing.T) {
@@ -1587,3 +1587,68 @@ func TestSetBytes(t *testing.T) {
 		t.Errorf("ValueOf(%p).Bytes() = %p", &x[0], &y[0])
 	}
 }
+
+type Private struct {
+	x int
+	y **int
+}
+
+func (p *Private) m() {
+}
+
+type Public struct {
+	X int
+	Y **int
+}
+
+func (p *Public) M() {
+}
+
+func TestUnexported(t *testing.T) {
+	var pub Public
+	v := ValueOf(&pub)
+	isValid(v.Elem().Field(0))
+	isValid(v.Elem().Field(1))
+	isValid(v.Elem().FieldByName("X"))
+	isValid(v.Elem().FieldByName("Y"))
+	isValid(v.Type().Method(0).Func)
+	isNonNil(v.Elem().Field(0).Interface())
+	isNonNil(v.Elem().Field(1).Interface())
+	isNonNil(v.Elem().FieldByName("X").Interface())
+	isNonNil(v.Elem().FieldByName("Y").Interface())
+	isNonNil(v.Type().Method(0).Func.Interface())
+
+	var priv Private
+	v = ValueOf(&priv)
+	isValid(v.Elem().Field(0))
+	isValid(v.Elem().Field(1))
+	isValid(v.Elem().FieldByName("x"))
+	isValid(v.Elem().FieldByName("y"))
+	isValid(v.Type().Method(0).Func)
+	shouldPanic(func() { v.Elem().Field(0).Interface() })
+	shouldPanic(func() { v.Elem().Field(1).Interface() })
+	shouldPanic(func() { v.Elem().FieldByName("x").Interface() })
+	shouldPanic(func() { v.Elem().FieldByName("y").Interface() })
+	shouldPanic(func() { v.Type().Method(0).Func.Interface() })
+}
+
+func shouldPanic(f func()) {
+	defer func() {
+		if recover() == nil {
+			panic("did not panic")
+		}
+	}()
+	f()
+}
+
+func isNonNil(x interface{}) {
+	if x == nil {
+		panic("nil interface")
+	}
+}
+
+func isValid(v Value) {
+	if !v.IsValid() {
+		panic("zero Value")
+	}
+}