[release-branch.go1.5] fmt: in Scanf, %c can scan a space, so don't skip spaces at %c

In short, %c should just give you the next rune, period.
Apparently this is the design. I use the term loosely.

Fixes #12275

Change-Id: I6f30bed442c0e88eac2244d465c7d151b29cf393
Reviewed-on: https://go-review.googlesource.com/13821Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/14395

src/fmt/scan.go

@@ -83,6 +83,8 @@ func Scanln(a ...interface{}) (n int, err error) {
 // the format.  It returns the number of items successfully scanned.
 // If that is less than the number of arguments, err will report why.
 // Newlines in the input must match newlines in the format.
+// The one exception: the verb %c always scans the next rune in the
+// input, even if it is a space (or tab etc.) or newline.
 func Scanf(format string, a ...interface{}) (n int, err error) {
 	return Fscanf(os.Stdin, format, a...)
 }
@@ -1164,15 +1166,18 @@ func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err erro
 		if !widPresent {
 			s.maxWid = hugeWid
 		}
-		s.SkipSpace()
+
+		c, w := utf8.DecodeRuneInString(format[i:])
+		i += w
+
+		if c != 'c' {
+			s.SkipSpace()
+		}
 		s.argLimit = s.limit
 		if f := s.count + s.maxWid; f < s.argLimit {
 			s.argLimit = f
 		}
 
-		c, w := utf8.DecodeRuneInString(format[i:])
-		i += w
-
 		if numProcessed >= len(a) { // out of operands
 			s.errorString("too few operands for format %" + format[i-w:])
 			break

src/fmt/scan_test.go

@@ -300,10 +300,13 @@ var scanfTests = []ScanfTest{
 	{"%2s", "sssss", &xVal, Xs("ss")},
 
 	// Fixed bugs
-	{"%d\n", "27\n", &intVal, 27},  // ok
-	{"%d\n", "28 \n", &intVal, 28}, // was: "unexpected newline"
-	{"%v", "0", &intVal, 0},        // was: "EOF"; 0 was taken as base prefix and not counted.
-	{"%v", "0", &uintVal, uint(0)}, // was: "EOF"; 0 was taken as base prefix and not counted.
+	{"%d\n", "27\n", &intVal, 27},      // ok
+	{"%d\n", "28 \n", &intVal, 28},     // was: "unexpected newline"
+	{"%v", "0", &intVal, 0},            // was: "EOF"; 0 was taken as base prefix and not counted.
+	{"%v", "0", &uintVal, uint(0)},     // was: "EOF"; 0 was taken as base prefix and not counted.
+	{"%c", " ", &uintVal, uint(' ')},   // %c must accept a blank.
+	{"%c", "\t", &uintVal, uint('\t')}, // %c must accept any space.
+	{"%c", "\n", &uintVal, uint('\n')}, // %c must accept any space.
 }
 
 var overflowTests = []ScanTest{