net/mail: better error in ParseAddress when missing "@domain"

If the input was "John Doe", we're definitely missing "<email>", as
"John Doe@domain" isn't a valid email address.

However, if the input was "john.doe", it's possible that the user meant
"john.doe@domain", and not just "john.doe <email>". Make it clear in the
error that either could be the source of the problem.

Fixes #27064.

Change-Id: I1b8f1342775d711823dffc3db974898ee62d3a34
Reviewed-on: https://go-review.googlesource.com/c/go/+/165517
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/mail/message.go

@@ -342,6 +342,21 @@ func (p *addrParser) parseAddress(handleGroup bool) ([]*Address, error) {
 	}
 	// angle-addr = "<" addr-spec ">"
 	if !p.consume('<') {
+		atext := true
+		for _, r := range displayName {
+			if !isAtext(r, true, false) {
+				atext = false
+				break
+			}
+		}
+		if atext {
+			// The input is like "foo.bar"; it's possible the input
+			// meant to be "foo.bar@domain", or "foo.bar <...>".
+			return nil, errors.New("mail: missing '@' or angle-addr")
+		}
+		// The input is like "Full Name", which couldn't possibly be a
+		// valid email address if followed by "@domain"; the input
+		// likely meant to be "Full Name <...>".
 		return nil, errors.New("mail: no angle-addr")
 	}
 	spec, err = p.consumeAddrSpec()

src/net/mail/message_test.go

@@ -144,6 +144,9 @@ func TestAddressParsingError(t *testing.T) {
 		12: {"root group: embed group: null@example.com;", "no angle-addr"},
 		13: {"group not closed: null@example.com", "expected comma"},
 		14: {"group: first@example.com, second@example.com;", "group with multiple addresses"},
+		15: {"john.doe", "missing '@' or angle-addr"},
+		16: {"john.doe@", "no angle-addr"},
+		17: {"John Doe@foo.bar", "no angle-addr"},
 	}
 
 	for i, tc := range mustErrTestCases {