fmt: treat reflect.Value specially - as the value it holds

When a reflect.Value is passed to Printf (etc.), fmt called the
String method, which does not disclose its contents. To get the
contents, one could call Value.Interface(), but that is illegal
if the Value is not exported or otherwise forbidden.

This CL improves the situation with a trivial change to the
fmt package: when we see a reflect.Value as an argument,
we treat it exactly as we treat a reflect.Value we make inside
the package. This means that we always print the
contents of the Value as if _that_ was the argument to Printf.

This is arguably a breaking change but I think it is a genuine
improvement and no greater a break than many other tweaks
we have made to formatted output from this package.

Fixes #8965.

Change-Id: Ifc2a4ce3c1134ad5160e101d2196c22f1542faab
Reviewed-on: https://go-review.googlesource.com/8731Reviewed-by: roger peppe <rogpeppe@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/fmt/doc.go

@@ -138,20 +138,23 @@
 	formatting considerations apply for operands that implement
 	certain interfaces. In order of application:
 
-	1. If an operand implements the Formatter interface, it will
+	1. If the operand is a reflect.Value, the concrete value it
+	holds is printed as if it was the operand.
+
+	2. If an operand implements the Formatter interface, it will
 	be invoked. Formatter provides fine control of formatting.
 
-	2. If the %v verb is used with the # flag (%#v) and the operand
+	3. If the %v verb is used with the # flag (%#v) and the operand
 	implements the GoStringer interface, that will be invoked.
 
 	If the format (which is implicitly %v for Println etc.) is valid
 	for a string (%s %q %v %x %X), the following two rules apply:
 
-	3. If an operand implements the error interface, the Error method
+	4. If an operand implements the error interface, the Error method
 	will be invoked to convert the object to a string, which will then
 	be formatted as required by the verb (if any).
 
-	4. If an operand implements method String() string, that method
+	5. If an operand implements method String() string, that method
 	will be invoked to convert the object to a string, which will then
 	be formatted as required by the verb (if any).
 

src/fmt/fmt_test.go

@@ -9,6 +9,7 @@ import (
 	. "fmt"
 	"io"
 	"math"
+	"reflect"
 	"runtime"
 	"strings"
 	"testing"
@@ -679,6 +680,12 @@ var fmtTests = []struct {
 	{"%x", byteFormatterSlice, "61626364"},
 	// This next case seems wrong, but the docs say the Formatter wins here.
 	{"%#v", byteFormatterSlice, "[]fmt_test.byteFormatter{X, X, X, X}"},
+
+	// reflect.Value handled specially in Go 1.5, making it possible to
+	// see inside non-exported fields (which cannot be accessed with Interface()).
+	// Issue 8965.
+	{"%v", reflect.ValueOf(A{}).Field(0).String(), "<int Value>"}, // Equivalent to the old way.
+	{"%v", reflect.ValueOf(A{}).Field(0), "0"},                    // Sees inside the field.
 }
 
 // zeroFill generates zero-filled strings of the specified width. The length

src/fmt/print.go

@@ -789,6 +789,8 @@ func (p *pp) printArg(arg interface{}, verb rune, depth int) (wasString bool) {
 	case []byte:
 		p.fmtBytes(f, verb, nil, depth)
 		wasString = verb == 's'
+	case reflect.Value:
+		return p.printReflectValue(f, verb, depth)
 	default:
 		// If the type is not simple, it might have methods.
 		if handled := p.handleMethods(verb, depth); handled {

src/reflect/value.go

@@ -1622,6 +1622,8 @@ func (v Value) Slice3(i, j, k int) Value {
 // String is a special case because of Go's String method convention.
 // Unlike the other getters, it does not panic if v's Kind is not String.
 // Instead, it returns a string of the form "<T value>" where T is v's type.
+// The fmt package treats Values specially. It does not call their String
+// method implicitly but instead prints the concrete values they hold.
 func (v Value) String() string {
 	switch k := v.kind(); k {
 	case Invalid: