fmt: diagnose invalid verb applied to pointer

Fixes #2851.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5644048

src/pkg/fmt/fmt_test.go

@@ -423,6 +423,7 @@ var fmttests = []struct {
 	{"p0=%p", new(int), "p0=0xPTR"},
 	{"p1=%s", &pValue, "p1=String(p)"}, // String method...
 	{"p2=%p", &pValue, "p2=0xPTR"},     // ... not called with %p
+	{"p3=%p", (*int)(nil), "p3=0x0"},
 	{"p4=%#p", new(int), "p4=PTR"},
 
 	// %p on non-pointers
@@ -431,6 +432,14 @@ var fmttests = []struct {
 	{"%p", make([]int, 1), "0xPTR"},
 	{"%p", 27, "%!p(int=27)"}, // not a pointer at all
 
+	// %q on pointers
+	{"%q", (*int)(nil), "%!q(*int=<nil>)"},
+	{"%q", new(int), "%!q(*int=0xPTR)"},
+
+	// %v on pointers formats 0 as <nil>
+	{"%v", (*int)(nil), "<nil>"},
+	{"%v", new(int), "0xPTR"},
+
 	// %d on Stringer should give integer if possible
 	{"%s", time.Time{}.Month(), "January"},
 	{"%d", time.Time{}.Month(), "1"},

src/pkg/fmt/print.go

@@ -553,6 +553,14 @@ func (p *pp) fmtBytes(v []byte, verb rune, goSyntax bool, depth int) {
 }
 
 func (p *pp) fmtPointer(value reflect.Value, verb rune, goSyntax bool) {
+	switch verb {
+	case 'p', 'v', 'b', 'd', 'o', 'x', 'X':
+		// ok
+	default:
+		p.badVerb(verb)
+		return
+	}
+
 	var u uintptr
 	switch value.Kind() {
 	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.Slice, reflect.UnsafePointer:
@@ -561,6 +569,7 @@ func (p *pp) fmtPointer(value reflect.Value, verb rune, goSyntax bool) {
 		p.badVerb(verb)
 		return
 	}
+
 	if goSyntax {
 		p.add('(')
 		p.buf.WriteString(value.Type().String())
@@ -572,6 +581,8 @@ func (p *pp) fmtPointer(value reflect.Value, verb rune, goSyntax bool) {
 			p.fmt0x64(uint64(u), true)
 		}
 		p.add(')')
+	} else if verb == 'v' && u == 0 {
+		p.buf.Write(nilAngleBytes)
 	} else {
 		p.fmt0x64(uint64(u), !p.fmt.sharp)
 	}
@@ -929,24 +940,7 @@ BigSwitch:
 				break BigSwitch
 			}
 		}
-		if goSyntax {
-			p.buf.WriteByte('(')
-			p.buf.WriteString(value.Type().String())
-			p.buf.WriteByte(')')
-			p.buf.WriteByte('(')
-			if v == 0 {
-				p.buf.Write(nilBytes)
-			} else {
-				p.fmt0x64(uint64(v), true)
-			}
-			p.buf.WriteByte(')')
-			break
-		}
-		if v == 0 {
-			p.buf.Write(nilAngleBytes)
-			break
-		}
-		p.fmt0x64(uint64(v), true)
+		fallthrough
 	case reflect.Chan, reflect.Func, reflect.UnsafePointer:
 		p.fmtPointer(value, verb, goSyntax)
 	default: