fmt.Print: fix bug in placement of spaces introduced when ...T went in.

Fixes #976.

R=rsc, chris tighe, r
CC=golang-dev
https://golang.org/cl/1697057

src/pkg/fmt/fmt_test.go

@@ -526,3 +526,23 @@ func TestEmptyMap(t *testing.T) {
 		t.Errorf("empty map printed as %q not %q", s, emptyMapStr)
 	}
 }
+
+// Check that Sprint (and hence Print, Fprint) puts spaces in the right places,
+// that is, between arg pairs in which neither is a string.
+func TestBlank(t *testing.T) {
+	got := Sprint("<", 1, ">:", 1, 2, 3, "!")
+	expect := "<1>:1 2 3!"
+	if got != expect {
+		t.Errorf("got %q expected %q", got, expect)
+	}
+}
+
+// Check that Sprintln (and hence Println, Fprintln) puts spaces in the right places,
+// that is, between all arg pairs.
+func TestBlankln(t *testing.T) {
+	got := Sprintln("<", 1, ">:", 1, 2, 3, "!")
+	expect := "< 1 >: 1 2 3 !\n"
+	if got != expect {
+		t.Errorf("got %q expected %q", got, expect)
+	}
+}

src/pkg/fmt/print.go

@@ -492,7 +492,7 @@ var (
 	uintptrBits = reflect.Typeof(uintptr(0)).Bits()
 )
 
-func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth int) (was_string bool) {
+func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth int) (wasString bool) {
 	if field != nil {
 		switch {
 		default:
@@ -850,18 +850,18 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 }
 
 func (p *pp) doPrint(a []interface{}, addspace, addnewline bool) {
-	prev_string := false
+	prevString := false
 	for fieldnum := 0; fieldnum < len(a); fieldnum++ {
 		p.fmt.clearflags()
 		// always add spaces if we're doing println
 		field := a[fieldnum]
 		if fieldnum > 0 {
-			_, is_string := field.(*reflect.StringValue)
-			if addspace || !is_string && !prev_string {
+			isString := field != nil && reflect.Typeof(field).Kind() == reflect.String
+			if addspace || !isString && !prevString {
 				p.buf.WriteByte(' ')
 			}
 		}
-		prev_string = p.printField(field, 'v', false, false, 0)
+		prevString = p.printField(field, 'v', false, false, 0)
 	}
 	if addnewline {
 		p.buf.WriteByte('\n')