fmt: fix zero padding for NaN

Makes zero padding of NaN and infinities consistent
by using spaces instead of zeroes to pad NaN.
Adds more tests for NaN formatting.

Fixes #14421

Change-Id: Ia20f8e878cc81ac72a744ec10d65e84b94e09c6a
Reviewed-on: https://go-review.googlesource.com/19723Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -386,6 +386,9 @@ var fmtTests = []struct {
 	{"%20e", math.Inf(1), "                +Inf"},
 	{"%-20f", math.Inf(-1), "-Inf                "},
 	{"%20g", math.NaN(), "                 NaN"},
+	{"%+20f", math.NaN(), "                +NaN"},
+	{"% -20f", math.NaN(), " NaN                "},
+	{"%+-20f", math.NaN(), "+NaN                "},
 
 	// arrays
 	{"%v", array, "[1 2 3 4 5]"},
@@ -654,13 +657,19 @@ var fmtTests = []struct {
 	// Complex numbers: exhaustively tested in TestComplexFormatting.
 	{"%7.2f", 1 + 2i, "(   1.00  +2.00i)"},
 	{"%+07.2f", -1 - 2i, "(-001.00-002.00i)"},
-	// Zero padding does not apply to infinities.
+	// Zero padding does not apply to infinities and NaN.
 	{"%020f", math.Inf(-1), "                -Inf"},
 	{"%020f", math.Inf(+1), "                +Inf"},
+	{"%020f", math.NaN(), "                 NaN"},
 	{"% 020f", math.Inf(-1), "                -Inf"},
 	{"% 020f", math.Inf(+1), "                 Inf"},
+	{"% 020f", math.NaN(), "                 NaN"},
 	{"%+020f", math.Inf(-1), "                -Inf"},
 	{"%+020f", math.Inf(+1), "                +Inf"},
+	{"%+020f", math.NaN(), "                +NaN"},
+	{"%-020f", math.Inf(-1), "-Inf                "},
+	{"%-020f", math.Inf(+1), "+Inf                "},
+	{"%-020f", math.NaN(), "NaN                 "},
 	{"%20f", -1.0, "           -1.000000"},
 	// Make sure we can handle very large widths.
 	{"%0100f", -1.0, zeroFill("-", 99, "1.000000")},

src/fmt/format.go

@@ -414,11 +414,15 @@ func (f *fmt) formatFloat(v float64, verb byte, prec, n int) {
 	if f.space && num[0] == '+' {
 		num[0] = ' '
 	}
-	// Special handling for "+Inf" and "-Inf",
+	// Special handling for infinities and NaN,
 	// which don't look like a number so shouldn't be padded with zeros.
-	if num[1] == 'I' {
+	if num[1] == 'I' || num[1] == 'N' {
 		oldZero := f.zero
 		f.zero = false
+		// Remove sign before NaN if not asked for.
+		if num[1] == 'N' && !f.space && !f.plus {
+			num = num[1:]
+		}
 		f.pad(num)
 		f.zero = oldZero
 		return