fmt: always handle special methods if print operand is a reflect.Value

Check for and call the special printing and format methods such as String
at printing depth 0 when printing the concrete value of a reflect.Value.

Fixes: #16015

Change-Id: I23bd2927255b60924e5558321e98dd4a95e11c4c
Reviewed-on: https://go-review.googlesource.com/30753Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/fmt/fmt_test.go

@@ -605,7 +605,10 @@ var fmtTests = []struct {
 	{"%x", I(23), `3c32333e`},
 	{"%#x", I(23), `0x3c32333e`},
 	{"%# x", I(23), `0x3c 0x32 0x33 0x3e`},
-	{"%d", I(23), `23`}, // Stringer applies only to string formats.
+	// Stringer applies only to string formats.
+	{"%d", I(23), `23`},
+	// Stringer applies to the extracted value.
+	{"%s", reflect.ValueOf(I(23)), `<23>`},
 
 	// go syntax
 	{"%#v", A{1, 2, "a", []int{1, 2}}, `fmt_test.A{i:1, j:0x2, s:"a", x:[]int{1, 2}}`},

src/fmt/print.go

@@ -659,6 +659,14 @@ func (p *pp) printArg(arg interface{}, verb rune) {
 	case []byte:
 		p.fmtBytes(f, verb, "[]byte")
 	case reflect.Value:
+		// Handle extractable values with special methods
+		// since printValue does not handle them at depth 0.
+		if f.IsValid() && f.CanInterface() {
+			p.arg = f.Interface()
+			if p.handleMethods(verb) {
+				return
+			}
+		}
 		p.printValue(f, verb, 0)
 	default:
 		// If the type is not simple, it might have methods.