fmt: fix %v of complex slice

Fixes #4525.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6929049

src/pkg/fmt/fmt_test.go

@@ -476,6 +476,11 @@ var fmttests = []struct {
 
 	// Used to crash because nByte didn't allow for a sign.
 	{"%b", int64(-1 << 63), "-1000000000000000000000000000000000000000000000000000000000000000"},
+
+	// Complex fmt used to leave the plus flag set for future entries in the array
+	// causing +2+0i and +3+0i instead of 2+0i and 3+0i.
+	{"%v", []complex64{1, 2, 3}, "[(1+0i) (2+0i) (3+0i)]"},
+	{"%v", []complex128{1, 2, 3}, "[(1+0i) (2+0i) (3+0i)]"},
 }
 
 func TestSprintf(t *testing.T) {

src/pkg/fmt/format.go

@@ -428,6 +428,7 @@ func (f *fmt) fmt_fb32(v float32) { f.formatFloat(float64(v), 'b', 0, 32) }
 func (f *fmt) fmt_c64(v complex64, verb rune) {
 	f.buf.WriteByte('(')
 	r := real(v)
+	oldPlus := f.plus
 	for i := 0; ; i++ {
 		switch verb {
 		case 'e':
@@ -447,6 +448,7 @@ func (f *fmt) fmt_c64(v complex64, verb rune) {
 		f.plus = true
 		r = imag(v)
 	}
+	f.plus = oldPlus
 	f.buf.Write(irparenBytes)
 }
 
@@ -454,6 +456,7 @@ func (f *fmt) fmt_c64(v complex64, verb rune) {
 func (f *fmt) fmt_c128(v complex128, verb rune) {
 	f.buf.WriteByte('(')
 	r := real(v)
+	oldPlus := f.plus
 	for i := 0; ; i++ {
 		switch verb {
 		case 'e':
@@ -473,5 +476,6 @@ func (f *fmt) fmt_c128(v complex128, verb rune) {
 		f.plus = true
 		r = imag(v)
 	}
+	f.plus = oldPlus
 	f.buf.Write(irparenBytes)
 }