encoding/asn1: fix unmarshaling of implicitly tagged UTF-8 strings.

Fixes #8541.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/153770043

src/encoding/asn1/asn1.go

@@ -640,15 +640,19 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	// when it sees a string, so if we see a different string type on the
 	// wire, we change the universal type to match.
 	if universalTag == tagPrintableString {
-		switch t.tag {
-		case tagIA5String, tagGeneralString, tagT61String, tagUTF8String:
-			universalTag = t.tag
+		if params.tag == nil {
+			switch t.tag {
+			case tagIA5String, tagGeneralString, tagT61String, tagUTF8String:
+				universalTag = t.tag
+			}
+		} else if params.stringType != 0 {
+			universalTag = params.stringType
 		}
 	}
 
 	// Special case for time: UTCTime and GeneralizedTime both map to the
 	// Go type time.Time.
-	if universalTag == tagUTCTime && t.tag == tagGeneralizedTime {
+	if universalTag == tagUTCTime && params.tag == nil && t.tag == tagGeneralizedTime {
 		universalTag = tagGeneralizedTime
 	}
 

src/encoding/asn1/asn1_test.go

@@ -392,6 +392,10 @@ type TestContextSpecificTags2 struct {
 	B int
 }
 
+type TestContextSpecificTags3 struct {
+	S string `asn1:"tag:1,utf8"`
+}
+
 type TestElementsAfterString struct {
 	S    string
 	A, B int
@@ -420,6 +424,7 @@ var unmarshalTestData = []struct {
 	{[]byte{0x04, 0x04, 1, 2, 3, 4}, &RawValue{0, 4, false, []byte{1, 2, 3, 4}, []byte{4, 4, 1, 2, 3, 4}}},
 	{[]byte{0x30, 0x03, 0x81, 0x01, 0x01}, &TestContextSpecificTags{1}},
 	{[]byte{0x30, 0x08, 0xa1, 0x03, 0x02, 0x01, 0x01, 0x02, 0x01, 0x02}, &TestContextSpecificTags2{1, 2}},
+	{[]byte{0x30, 0x03, 0x81, 0x01, '@'}, &TestContextSpecificTags3{"@"}},
 	{[]byte{0x01, 0x01, 0x00}, newBool(false)},
 	{[]byte{0x01, 0x01, 0xff}, newBool(true)},
 	{[]byte{0x30, 0x0b, 0x13, 0x03, 0x66, 0x6f, 0x6f, 0x02, 0x01, 0x22, 0x02, 0x01, 0x33}, &TestElementsAfterString{"foo", 0x22, 0x33}},