text/template: catch (A).X as a parse error

This shouldn't be an error (see issue 3999), but until it's handled
correctly, treat it as one to avoid confusion. Without this CL,
(A).X parses as two arguments.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6473059

src/pkg/text/template/parse/lex.go

@@ -354,6 +354,12 @@ func lexInsideAction(l *lexer) stateFn {
 		if l.parenDepth < 0 {
 			return l.errorf("unexpected right paren %#U", r)
 		}
+		// Catch the mistake of (a).X, which will parse as two args.
+		// See issue 3999. TODO: Remove once arg parsing is
+		// better defined.
+		if l.peek() == '.' {
+			return l.errorf("cannot evaluate field of parenthesized expression")
+		}
 		return lexInsideAction
 	case r <= unicode.MaxASCII && unicode.IsPrint(r):
 		l.emit(itemChar)

src/pkg/text/template/parse/parse_test.go

@@ -232,6 +232,9 @@ var parseTests = []parseTest{
 	{"invalid punctuation", "{{printf 3, 4}}", hasError, ""},
 	{"multidecl outside range", "{{with $v, $u := 3}}{{end}}", hasError, ""},
 	{"too many decls in range", "{{range $u, $v, $w := 3}}{{end}}", hasError, ""},
+	// This one should work but doesn't. Caught as a parse error to avoid confusion.
+	// TODO: Update after issue 3999 is resolved.
+	{"dot applied to parentheses", "{{printf (printf .).}}", hasError, ""},
 	// Equals (and other chars) do not assignments make (yet).
 	{"bug0a", "{{$x := 0}}{{$x}}", noError, "{{$x := 0}}{{$x}}"},
 	{"bug0b", "{{$x = 1}}{{$x}}", hasError, ""},