fmt: Add support for capital '%X' format verb for scanning

For printing, the format verb '%X' results in a capitalized
hex-representation of the formatted value. Conversely, using
'%X' in a Scanf function should scan a hex-representation
into the given interface{}. The existing implementation
however only supports '%X' for scanning hex values into
integers; strings or byte slices remain empty. On the other
hand, lower-case '%x' supports strings and byte slices just
fine. This is merely an oversight, which this commit fixes.
(Additional tests also included.)

    Fixes #12940

Change-Id: I178a7f615bae950dfc014ca8c0a038448cf0452a
Reviewed-on: https://go-review.googlesource.com/15689Reviewed-by: Andrew Gerrand <adg@golang.org>

src/fmt/scan.go

@@ -813,7 +813,7 @@ func (s *ss) scanComplex(verb rune, n int) complex128 {
 // convertString returns the string represented by the next input characters.
 // The format of the input is determined by the verb.
 func (s *ss) convertString(verb rune) (str string) {
-	if !s.okVerb(verb, "svqx", "string") {
+	if !s.okVerb(verb, "svqxX", "string") {
 		return ""
 	}
 	s.skipSpace(false)
@@ -821,7 +821,7 @@ func (s *ss) convertString(verb rune) (str string) {
 	switch verb {
 	case 'q':
 		str = s.quotedString()
-	case 'x':
+	case 'x', 'X':
 		str = s.hexString()
 	default:
 		str = string(s.token(true, notSpace)) // %s and %v just return the next word

src/fmt/scan_test.go

@@ -255,12 +255,14 @@ var scanfTests = []ScanfTest{
 	// Strings
 	{"%s", "using-%s\n", &stringVal, "using-%s"},
 	{"%x", "7573696e672d2578\n", &stringVal, "using-%x"},
+	{"%X", "7573696E672D2558\n", &stringVal, "using-%X"},
 	{"%q", `"quoted\twith\\do\u0075bl\x65s"` + "\n", &stringVal, "quoted\twith\\doubles"},
 	{"%q", "`quoted with backs`\n", &stringVal, "quoted with backs"},
 
 	// Byte slices
 	{"%s", "bytes-%s\n", &bytesVal, []byte("bytes-%s")},
 	{"%x", "62797465732d2578\n", &bytesVal, []byte("bytes-%x")},
+	{"%X", "62797465732D2558\n", &bytesVal, []byte("bytes-%X")},
 	{"%q", `"bytes\rwith\vdo\u0075bl\x65s"` + "\n", &bytesVal, []byte("bytes\rwith\vdoubles")},
 	{"%q", "`bytes with backs`\n", &bytesVal, []byte("bytes with backs")},