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/fmt/fmt_test.go

@@ -62,7 +62,7 @@ type I int
 func (i I) String() string { return Sprintf("<%d>", int(i)) }
 
 type B struct {
-	i I
+	I I
 	j int
 }
 
@@ -84,8 +84,8 @@ func (g G) GoString() string {
 }
 
 type S struct {
-	f F // a struct field that Formats
+	F F // a struct field that Formats
-	g G // a struct field that GoStrings
+	G G // a struct field that GoStrings
 }
 
 // A type with a String method with pointer receiver for testing %p
@@ -333,8 +333,8 @@ var fmttests = []struct {
 	{"%+v", A{1, 2, "a", []int{1, 2}}, `{i:1 j:2 s:a x:[1 2]}`},
 
 	// +v on structs with Stringable items
-	{"%+v", B{1, 2}, `{i:<1> j:2}`},
+	{"%+v", B{1, 2}, `{I:<1> j:2}`},
-	{"%+v", C{1, B{2, 3}}, `{i:1 B:{i:<2> j:3}}`},
+	{"%+v", C{1, B{2, 3}}, `{i:1 B:{I:<2> j:3}}`},
 
 	// q on Stringable items
 	{"%s", I(23), `<23>`},
@@ -350,7 +350,7 @@ var fmttests = []struct {
 	{"%#v", uint64(1<<64 - 1), "0xffffffffffffffff"},
 	{"%#v", 1000000000, "1000000000"},
 	{"%#v", map[string]int{"a": 1, "b": 2}, `map[string] int{"a":1, "b":2}`},
-	{"%#v", map[string]B{"a": {1, 2}, "b": {3, 4}}, `map[string] fmt_test.B{"a":fmt_test.B{i:1, j:2}, "b":fmt_test.B{i:3, j:4}}`},
+	{"%#v", map[string]B{"a": {1, 2}, "b": {3, 4}}, `map[string] fmt_test.B{"a":fmt_test.B{I:1, j:2}, "b":fmt_test.B{I:3, j:4}}`},
 	{"%#v", []string{"a", "b"}, `[]string{"a", "b"}`},
 
 	// slices with other formats
@@ -385,11 +385,11 @@ var fmttests = []struct {
 	// Formatter
 	{"%x", F(1), "<x=F(1)>"},
 	{"%x", G(2), "2"},
-	{"%+v", S{F(4), G(5)}, "{f:<v=F(4)> g:5}"},
+	{"%+v", S{F(4), G(5)}, "{F:<v=F(4)> G:5}"},
 
 	// GoStringer
 	{"%#v", G(6), "GoString(6)"},
-	{"%#v", S{F(7), G(8)}, "fmt_test.S{f:<v=F(7)>, g:GoString(8)}"},
+	{"%#v", S{F(7), G(8)}, "fmt_test.S{F:<v=F(7)>, G:GoString(8)}"},
 
 	// %T
 	{"%T", (4 - 3i), "complex128"},

src/pkg/fmt/print.go

@@ -262,10 +262,8 @@ func Sprintln(a ...interface{}) string {
 // the thing inside the interface, not the interface itself.
 func getField(v reflect.Value, i int) reflect.Value {
 	val := v.Field(i)
-	if i := val; i.Kind() == reflect.Interface {
+	if val.Kind() == reflect.Interface && !val.IsNil() {
-		if inter := i.Interface(); inter != nil {
+		val = val.Elem()
-			return reflect.ValueOf(inter)
-		}
 	}
 	return val
 }
@@ -292,27 +290,32 @@ func (p *pp) unknownType(v interface{}) {
 	p.buf.WriteByte('?')
 }
 
-func (p *pp) badVerb(verb int, val interface{}) {
+func (p *pp) badVerb(verb int, val interface{}, val1 reflect.Value) {
 	p.add('%')
 	p.add('!')
 	p.add(verb)
 	p.add('(')
-	if val == nil {
+	switch {
-		p.buf.Write(nilAngleBytes)
+	case val != nil:
-	} else {
 		p.buf.WriteString(reflect.TypeOf(val).String())
 		p.add('=')
 		p.printField(val, 'v', false, false, 0)
+	case val1.IsValid():
+		p.buf.WriteString(val1.Type().String())
+		p.add('=')
+		p.printValue(val1, 'v', false, false, 0)
+	default:
+		p.buf.Write(nilAngleBytes)
 	}
 	p.add(')')
 }
 
-func (p *pp) fmtBool(v bool, verb int, value interface{}) {
+func (p *pp) fmtBool(v bool, verb int, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 't', 'v':
 		p.fmt.fmt_boolean(v)
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
@@ -326,7 +329,7 @@ func (p *pp) fmtC(c int64) {
 	p.fmt.pad(p.runeBuf[0:w])
 }
 
-func (p *pp) fmtInt64(v int64, verb int, value interface{}) {
+func (p *pp) fmtInt64(v int64, verb int, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'b':
 		p.fmt.integer(v, 2, signed, ldigits)
@@ -340,7 +343,7 @@ func (p *pp) fmtInt64(v int64, verb int, value interface{}) {
 		if 0 <= v && v <= unicode.MaxRune {
 			p.fmt.fmt_qc(v)
 		} else {
-			p.badVerb(verb, value)
+			p.badVerb(verb, value, value1)
 		}
 	case 'x':
 		p.fmt.integer(v, 16, signed, ldigits)
@@ -349,7 +352,7 @@ func (p *pp) fmtInt64(v int64, verb int, value interface{}) {
 	case 'X':
 		p.fmt.integer(v, 16, signed, udigits)
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
@@ -384,7 +387,7 @@ func (p *pp) fmtUnicode(v int64) {
 	p.fmt.sharp = sharp
 }
 
-func (p *pp) fmtUint64(v uint64, verb int, goSyntax bool, value interface{}) {
+func (p *pp) fmtUint64(v uint64, verb int, goSyntax bool, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'b':
 		p.fmt.integer(int64(v), 2, unsigned, ldigits)
@@ -404,7 +407,7 @@ func (p *pp) fmtUint64(v uint64, verb int, goSyntax bool, value interface{}) {
 		if 0 <= v && v <= unicode.MaxRune {
 			p.fmt.fmt_qc(int64(v))
 		} else {
-			p.badVerb(verb, value)
+			p.badVerb(verb, value, value1)
 		}
 	case 'x':
 		p.fmt.integer(int64(v), 16, unsigned, ldigits)
@@ -413,11 +416,11 @@ func (p *pp) fmtUint64(v uint64, verb int, goSyntax bool, value interface{}) {
 	case 'U':
 		p.fmtUnicode(int64(v))
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
-func (p *pp) fmtFloat32(v float32, verb int, value interface{}) {
+func (p *pp) fmtFloat32(v float32, verb int, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'b':
 		p.fmt.fmt_fb32(v)
@@ -432,11 +435,11 @@ func (p *pp) fmtFloat32(v float32, verb int, value interface{}) {
 	case 'G':
 		p.fmt.fmt_G32(v)
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
-func (p *pp) fmtFloat64(v float64, verb int, value interface{}) {
+func (p *pp) fmtFloat64(v float64, verb int, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'b':
 		p.fmt.fmt_fb64(v)
@@ -451,33 +454,33 @@ func (p *pp) fmtFloat64(v float64, verb int, value interface{}) {
 	case 'G':
 		p.fmt.fmt_G64(v)
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
-func (p *pp) fmtComplex64(v complex64, verb int, value interface{}) {
+func (p *pp) fmtComplex64(v complex64, verb int, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'e', 'E', 'f', 'F', 'g', 'G':
 		p.fmt.fmt_c64(v, verb)
 	case 'v':
 		p.fmt.fmt_c64(v, 'g')
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
-func (p *pp) fmtComplex128(v complex128, verb int, value interface{}) {
+func (p *pp) fmtComplex128(v complex128, verb int, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'e', 'E', 'f', 'F', 'g', 'G':
 		p.fmt.fmt_c128(v, verb)
 	case 'v':
 		p.fmt.fmt_c128(v, 'g')
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
-func (p *pp) fmtString(v string, verb int, goSyntax bool, value interface{}) {
+func (p *pp) fmtString(v string, verb int, goSyntax bool, value interface{}, value1 reflect.Value) {
 	switch verb {
 	case 'v':
 		if goSyntax {
@@ -494,11 +497,11 @@ func (p *pp) fmtString(v string, verb int, goSyntax bool, value interface{}) {
 	case 'q':
 		p.fmt.fmt_q(v)
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
-func (p *pp) fmtBytes(v []byte, verb int, goSyntax bool, depth int, value interface{}) {
+func (p *pp) fmtBytes(v []byte, verb int, goSyntax bool, depth int, value interface{}, value1 reflect.Value) {
 	if verb == 'v' || verb == 'd' {
 		if goSyntax {
 			p.buf.Write(bytesBytes)
@@ -533,7 +536,7 @@ func (p *pp) fmtBytes(v []byte, verb int, goSyntax bool, depth int, value interf
 	case 'q':
 		p.fmt.fmt_q(s)
 	default:
-		p.badVerb(verb, value)
+		p.badVerb(verb, value, value1)
 	}
 }
 
@@ -543,12 +546,12 @@ func (p *pp) fmtPointer(field interface{}, value reflect.Value, verb int, goSynt
 	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.Slice, reflect.UnsafePointer:
 		u = value.Pointer()
 	default:
-		p.badVerb(verb, field)
+		p.badVerb(verb, field, value)
 		return
 	}
 	if goSyntax {
 		p.add('(')
-		p.buf.WriteString(reflect.TypeOf(field).String())
+		p.buf.WriteString(value.Type().String())
 		p.add(')')
 		p.add('(')
 		if u == 0 {
@@ -594,12 +597,51 @@ func (p *pp) catchPanic(val interface{}, verb int) {
 	}
 }
 
+func (p *pp) handleMethods(field interface{}, verb int, plus, goSyntax bool, depth int) (wasString, handled bool) {
+	// Is it a Formatter?
+	if formatter, ok := field.(Formatter); ok {
+		handled = true
+		wasString = false
+		defer p.catchPanic(field, verb)
+		formatter.Format(p, verb)
+		return
+	}
+	// Must not touch flags before Formatter looks at them.
+	if plus {
+		p.fmt.plus = false
+	}
+
+	// If we're doing Go syntax and the field knows how to supply it, take care of it now.
+	if goSyntax {
+		p.fmt.sharp = false
+		if stringer, ok := field.(GoStringer); ok {
+			wasString = false
+			handled = true
+			defer p.catchPanic(field, verb)
+			// Print the result of GoString unadorned.
+			p.fmtString(stringer.GoString(), 's', false, field, reflect.Value{})
+			return
+		}
+	} else {
+		// Is it a Stringer?
+		if stringer, ok := field.(Stringer); ok {
+			wasString = false
+			handled = true
+			defer p.catchPanic(field, verb)
+			p.printField(stringer.String(), verb, plus, false, depth)
+			return
+		}
+	}
+	handled = false
+	return
+}
+
 func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth int) (wasString bool) {
 	if field == nil {
 		if verb == 'T' || verb == 'v' {
 			p.buf.Write(nilAngleBytes)
 		} else {
-			p.badVerb(verb, field)
+			p.badVerb(verb, field, reflect.Value{})
 		}
 		return false
 	}
@@ -614,118 +656,133 @@ func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth
 		p.fmtPointer(field, reflect.ValueOf(field), verb, goSyntax)
 		return false
 	}
-	// Is it a Formatter?
-	if formatter, ok := field.(Formatter); ok {
-		defer p.catchPanic(field, verb)
-		formatter.Format(p, verb)
-		return false // this value is not a string
 
-	}
+	if wasString, handled := p.handleMethods(field, verb, plus, goSyntax, depth); handled {
-	// Must not touch flags before Formatter looks at them.
+		return wasString
-	if plus {
-		p.fmt.plus = false
-	}
-	// If we're doing Go syntax and the field knows how to supply it, take care of it now.
-	if goSyntax {
-		p.fmt.sharp = false
-		if stringer, ok := field.(GoStringer); ok {
-			defer p.catchPanic(field, verb)
-			// Print the result of GoString unadorned.
-			p.fmtString(stringer.GoString(), 's', false, field)
-			return false // this value is not a string
-		}
-	} else {
-		// Is it a Stringer?
-		if stringer, ok := field.(Stringer); ok {
-			defer p.catchPanic(field, verb)
-			p.printField(stringer.String(), verb, plus, false, depth)
-			return false // this value is not a string
-		}
 	}
 
 	// Some types can be done without reflection.
 	switch f := field.(type) {
 	case bool:
-		p.fmtBool(f, verb, field)
+		p.fmtBool(f, verb, field, reflect.Value{})
 		return false
 	case float32:
-		p.fmtFloat32(f, verb, field)
+		p.fmtFloat32(f, verb, field, reflect.Value{})
 		return false
 	case float64:
-		p.fmtFloat64(f, verb, field)
+		p.fmtFloat64(f, verb, field, reflect.Value{})
 		return false
 	case complex64:
-		p.fmtComplex64(complex64(f), verb, field)
+		p.fmtComplex64(complex64(f), verb, field, reflect.Value{})
 		return false
 	case complex128:
-		p.fmtComplex128(f, verb, field)
+		p.fmtComplex128(f, verb, field, reflect.Value{})
 		return false
 	case int:
-		p.fmtInt64(int64(f), verb, field)
+		p.fmtInt64(int64(f), verb, field, reflect.Value{})
 		return false
 	case int8:
-		p.fmtInt64(int64(f), verb, field)
+		p.fmtInt64(int64(f), verb, field, reflect.Value{})
 		return false
 	case int16:
-		p.fmtInt64(int64(f), verb, field)
+		p.fmtInt64(int64(f), verb, field, reflect.Value{})
 		return false
 	case int32:
-		p.fmtInt64(int64(f), verb, field)
+		p.fmtInt64(int64(f), verb, field, reflect.Value{})
 		return false
 	case int64:
-		p.fmtInt64(f, verb, field)
+		p.fmtInt64(f, verb, field, reflect.Value{})
 		return false
 	case uint:
-		p.fmtUint64(uint64(f), verb, goSyntax, field)
+		p.fmtUint64(uint64(f), verb, goSyntax, field, reflect.Value{})
 		return false
 	case uint8:
-		p.fmtUint64(uint64(f), verb, goSyntax, field)
+		p.fmtUint64(uint64(f), verb, goSyntax, field, reflect.Value{})
 		return false
 	case uint16:
-		p.fmtUint64(uint64(f), verb, goSyntax, field)
+		p.fmtUint64(uint64(f), verb, goSyntax, field, reflect.Value{})
 		return false
 	case uint32:
-		p.fmtUint64(uint64(f), verb, goSyntax, field)
+		p.fmtUint64(uint64(f), verb, goSyntax, field, reflect.Value{})
 		return false
 	case uint64:
-		p.fmtUint64(f, verb, goSyntax, field)
+		p.fmtUint64(f, verb, goSyntax, field, reflect.Value{})
 		return false
 	case uintptr:
-		p.fmtUint64(uint64(f), verb, goSyntax, field)
+		p.fmtUint64(uint64(f), verb, goSyntax, field, reflect.Value{})
 		return false
 	case string:
-		p.fmtString(f, verb, goSyntax, field)
+		p.fmtString(f, verb, goSyntax, field, reflect.Value{})
 		return verb == 's' || verb == 'v'
 	case []byte:
-		p.fmtBytes(f, verb, goSyntax, depth, field)
+		p.fmtBytes(f, verb, goSyntax, depth, field, reflect.Value{})
 		return verb == 's'
 	}
 
 	// Need to use reflection
-	value := reflect.ValueOf(field)
+	return p.printReflectValue(reflect.ValueOf(field), verb, plus, goSyntax, depth)
+}
 
+// printValue is like printField but starts with a reflect value, not an interface{} value.
+func (p *pp) printValue(value reflect.Value, verb int, plus, goSyntax bool, depth int) (wasString bool) {
+	if !value.IsValid() {
+		if verb == 'T' || verb == 'v' {
+			p.buf.Write(nilAngleBytes)
+		} else {
+			p.badVerb(verb, nil, value)
+		}
+		return false
+	}
+
+	// Special processing considerations.
+	// %T (the value's type) and %p (its address) are special; we always do them first.
+	switch verb {
+	case 'T':
+		p.printField(value.Type().String(), 's', false, false, 0)
+		return false
+	case 'p':
+		p.fmtPointer(nil, value, verb, goSyntax)
+		return false
+	}
+
+	// Handle values with special methods.
+	// Call always, even when field == nil, because handleMethods clears p.fmt.plus for us.
+	var field interface{}
+	if value.CanInterface() {
+		field = value.Interface()
+	}
+	if wasString, handled := p.handleMethods(field, verb, plus, goSyntax, depth); handled {
+		return wasString
+	}
+
+	return p.printReflectValue(value, verb, plus, goSyntax, depth)
+}
+
+// printReflectValue is the fallback for both printField and printValue.
+// It uses reflect to print the value.
+func (p *pp) printReflectValue(value reflect.Value, verb int, plus, goSyntax bool, depth int) (wasString bool) {
 BigSwitch:
 	switch f := value; f.Kind() {
 	case reflect.Bool:
-		p.fmtBool(f.Bool(), verb, field)
+		p.fmtBool(f.Bool(), verb, nil, value)
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
-		p.fmtInt64(f.Int(), verb, field)
+		p.fmtInt64(f.Int(), verb, nil, value)
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
-		p.fmtUint64(uint64(f.Uint()), verb, goSyntax, field)
+		p.fmtUint64(uint64(f.Uint()), verb, goSyntax, nil, value)
 	case reflect.Float32, reflect.Float64:
 		if f.Type().Size() == 4 {
-			p.fmtFloat32(float32(f.Float()), verb, field)
+			p.fmtFloat32(float32(f.Float()), verb, nil, value)
 		} else {
-			p.fmtFloat64(float64(f.Float()), verb, field)
+			p.fmtFloat64(float64(f.Float()), verb, nil, value)
 		}
 	case reflect.Complex64, reflect.Complex128:
 		if f.Type().Size() == 8 {
-			p.fmtComplex64(complex64(f.Complex()), verb, field)
+			p.fmtComplex64(complex64(f.Complex()), verb, nil, value)
 		} else {
-			p.fmtComplex128(complex128(f.Complex()), verb, field)
+			p.fmtComplex128(complex128(f.Complex()), verb, nil, value)
 		}
 	case reflect.String:
-		p.fmtString(f.String(), verb, goSyntax, field)
+		p.fmtString(f.String(), verb, goSyntax, nil, value)
 	case reflect.Map:
 		if goSyntax {
 			p.buf.WriteString(f.Type().String())
@@ -742,9 +799,9 @@ BigSwitch:
 					p.buf.WriteByte(' ')
 				}
 			}
-			p.printField(key.Interface(), verb, plus, goSyntax, depth+1)
+			p.printValue(key, verb, plus, goSyntax, depth+1)
 			p.buf.WriteByte(':')
-			p.printField(f.MapIndex(key).Interface(), verb, plus, goSyntax, depth+1)
+			p.printValue(f.MapIndex(key), verb, plus, goSyntax, depth+1)
 		}
 		if goSyntax {
 			p.buf.WriteByte('}')
@@ -753,7 +810,7 @@ BigSwitch:
 		}
 	case reflect.Struct:
 		if goSyntax {
-			p.buf.WriteString(reflect.TypeOf(field).String())
+			p.buf.WriteString(value.Type().String())
 		}
 		p.add('{')
 		v := f
@@ -772,20 +829,20 @@ BigSwitch:
 					p.buf.WriteByte(':')
 				}
 			}
-			p.printField(getField(v, i).Interface(), verb, plus, goSyntax, depth+1)
+			p.printValue(getField(v, i), verb, plus, goSyntax, depth+1)
 		}
 		p.buf.WriteByte('}')
 	case reflect.Interface:
 		value := f.Elem()
 		if !value.IsValid() {
 			if goSyntax {
-				p.buf.WriteString(reflect.TypeOf(field).String())
+				p.buf.WriteString(value.Type().String())
 				p.buf.Write(nilParenBytes)
 			} else {
 				p.buf.Write(nilAngleBytes)
 			}
 		} else {
-			return p.printField(value.Interface(), verb, plus, goSyntax, depth+1)
+			return p.printValue(value, verb, plus, goSyntax, depth+1)
 		}
 	case reflect.Array, reflect.Slice:
 		// Byte slices are special.
@@ -801,11 +858,11 @@ BigSwitch:
 			for i := range bytes {
 				bytes[i] = byte(f.Index(i).Uint())
 			}
-			p.fmtBytes(bytes, verb, goSyntax, depth, field)
+			p.fmtBytes(bytes, verb, goSyntax, depth, nil, value)
 			return verb == 's'
 		}
 		if goSyntax {
-			p.buf.WriteString(reflect.TypeOf(field).String())
+			p.buf.WriteString(value.Type().String())
 			p.buf.WriteByte('{')
 		} else {
 			p.buf.WriteByte('[')
@@ -818,7 +875,7 @@ BigSwitch:
 					p.buf.WriteByte(' ')
 				}
 			}
-			p.printField(f.Index(i).Interface(), verb, plus, goSyntax, depth+1)
+			p.printValue(f.Index(i), verb, plus, goSyntax, depth+1)
 		}
 		if goSyntax {
 			p.buf.WriteByte('}')
@@ -833,17 +890,17 @@ BigSwitch:
 			switch a := f.Elem(); a.Kind() {
 			case reflect.Array, reflect.Slice:
 				p.buf.WriteByte('&')
-				p.printField(a.Interface(), verb, plus, goSyntax, depth+1)
+				p.printValue(a, verb, plus, goSyntax, depth+1)
 				break BigSwitch
 			case reflect.Struct:
 				p.buf.WriteByte('&')
-				p.printField(a.Interface(), verb, plus, goSyntax, depth+1)
+				p.printValue(a, verb, plus, goSyntax, depth+1)
 				break BigSwitch
 			}
 		}
 		if goSyntax {
 			p.buf.WriteByte('(')
-			p.buf.WriteString(reflect.TypeOf(field).String())
+			p.buf.WriteString(value.Type().String())
 			p.buf.WriteByte(')')
 			p.buf.WriteByte('(')
 			if v == 0 {
@@ -860,7 +917,7 @@ BigSwitch:
 		}
 		p.fmt0x64(uint64(v), true)
 	case reflect.Chan, reflect.Func, reflect.UnsafePointer:
-		p.fmtPointer(field, value, verb, goSyntax)
+		p.fmtPointer(nil, value, verb, goSyntax)
 	default:
 		p.unknownType(f)
 	}

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{}) {
+	if v != s.W.(interface{}) {
-		t.Error("Interface() on interface: ", v, s.w)
+		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},
+	{struct{}{}, "Foo", nil, 0},
-	{S0{a: 'a'}, "a", []int{0}, 'a'},
+	{S0{A: 'a'}, "A", []int{0}, 'a'},
-	{S0{}, "d", nil, 0},
+	{S0{}, "D", nil, 0},
-	{S1{S0: S0{a: 'a'}}, "a", []int{1, 0}, 'a'},
+	{S1{S0: S0{A: 'a'}}, "A", []int{1, 0}, 'a'},
-	{S1{b: 'b'}, "b", []int{0}, 'b'},
+	{S1{B: 'b'}, "B", []int{0}, 'b'},
 	{S1{}, "S0", []int{1}, 0},
-	{S1{S0: S0{c: 'c'}}, "c", []int{1, 2}, 'c'},
+	{S1{S0: S0{C: 'c'}}, "C", []int{1, 2}, 'c'},
-	{S2{a: 'a'}, "a", []int{0}, 'a'},
+	{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{B: 'b'}}, "B", []int{1, 0}, 'b'},
-	{S2{S1: &S1{S0: S0{c: 'c'}}}, "c", []int{1, 1, 2}, 'c'},
+	{S2{S1: &S1{S0: S0{C: 'c'}}}, "C", []int{1, 1, 2}, 'c'},
-	{S2{}, "d", nil, 0},
+	{S2{}, "D", nil, 0},
 	{S3{}, "S1", nil, 0},
-	{S3{S2: S2{a: 'a'}}, "a", []int{1, 0}, 'a'},
+	{S3{S2: S2{A: 'a'}}, "A", []int{1, 0}, 'a'},
-	{S3{}, "b", nil, 0},
+	{S3{}, "B", nil, 0},
-	{S3{d: 'd'}, "d", []int{2}, 0},
+	{S3{D: 'd'}, "D", []int{2}, 0},
-	{S3{e: 'e'}, "e", []int{3}, 'e'},
+	{S3{E: 'e'}, "E", []int{3}, 'e'},
-	{S4{a: 'a'}, "a", []int{1}, 'a'},
+	{S4{A: 'a'}, "A", []int{1}, 'a'},
-	{S4{}, "b", nil, 0},
+	{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")
+	}
+}

src/pkg/reflect/deepequal.go

@@ -104,7 +104,7 @@ func deepValueEqual(v1, v2 Value, visited map[uintptr]*visit, depth int) (b bool
 		return true
 	default:
 		// Normal equality suffices
-		return v1.Interface() == v2.Interface()
+		return valueInterface(v1, false) == valueInterface(v2, false)
 	}
 
 	panic("Not reached")

src/pkg/reflect/value.go

@@ -876,14 +876,7 @@ func (v Value) CanInterface() bool {
 	if iv.kind == Invalid {
 		panic(&ValueError{"reflect.Value.CanInterface", iv.kind})
 	}
-	// TODO(rsc): Check flagRO too.  Decide what to do about asking for
+	return v.InternalMethod == 0 && iv.flag&flagRO == 0
-	// interface for a value obtained via an unexported field.
-	// If the field were of a known type, say chan int or *sync.Mutex,
-	// the caller could interfere with the data after getting the
-	// interface.  But fmt.Print depends on being able to look.
-	// Now that reflect is more efficient the special cases in fmt
-	// might be less important.
-	return v.InternalMethod == 0
 }
 
 // Interface returns v's value as an interface{}.
@@ -891,22 +884,28 @@ func (v Value) CanInterface() bool {
 // (as opposed to Type.Method), Interface cannot return an
 // interface value, so it panics.
 func (v Value) Interface() interface{} {
-	return v.internal().Interface()
+	return valueInterface(v, true)
 }
 
-func (iv internalValue) Interface() interface{} {
+func valueInterface(v Value, safe bool) interface{} {
+	iv := v.internal()
+	return iv.valueInterface(safe)
+}
+
+func (iv internalValue) valueInterface(safe bool) interface{} {
 	if iv.kind == 0 {
 		panic(&ValueError{"reflect.Value.Interface", iv.kind})
 	}
 	if iv.method {
 		panic("reflect.Value.Interface: cannot create interface value for method with bound receiver")
 	}
-	/*
-		if v.flag()&noExport != 0 {
-			panic("reflect.Value.Interface: cannot return value obtained from unexported struct field")
-		}
-	*/
 
+	if safe && iv.flag&flagRO != 0 {
+		// Do not allow access to unexported values via Interface,
+		// because they might be pointers that should not be 
+		// writable or methods or function that should not be callable.
+		panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
+	}
 	if iv.kind == Interface {
 		// Special case: return the element inside the interface.
 		// Won't recurse further because an interface cannot contain an interface.
@@ -1758,7 +1757,7 @@ func convertForAssignment(what string, addr unsafe.Pointer, dst Type, iv interna
 		if addr == nil {
 			addr = unsafe.Pointer(new(interface{}))
 		}
-		x := iv.Interface()
+		x := iv.valueInterface(false)
 		if dst.NumMethod() == 0 {
 			*(*interface{})(addr) = x
 		} else {

test/interface/fake.go

@@ -12,20 +12,20 @@ package main
 import "reflect"
 
 type T struct {
-	f float32
+	F float32
-	g float32
+	G float32
 
-	s string
+	S string
-	t string
+	T string
 
-	u uint32
+	U uint32
-	v uint32
+	V uint32
 
-	w uint32
+	W uint32
-	x uint32
+	X uint32
 
-	y uint32
+	Y uint32
-	z uint32
+	Z uint32
 }
 
 func add(s, t string) string {
@@ -40,16 +40,16 @@ func assert(b bool) {
 
 func main() {
 	var x T
-	x.f = 1.0
+	x.F = 1.0
-	x.g = x.f
+	x.G = x.F
-	x.s = add("abc", "def")
+	x.S = add("abc", "def")
-	x.t = add("abc", "def")
+	x.T = add("abc", "def")
-	x.u = 1
+	x.U = 1
-	x.v = 2
+	x.V = 2
-	x.w = 1 << 28
+	x.W = 1 << 28
-	x.x = 2 << 28
+	x.X = 2 << 28
-	x.y = 0x12345678
+	x.Y = 0x12345678
-	x.z = x.y
+	x.Z = x.Y
 
 	// check mem and string
 	v := reflect.ValueOf(x)