text/template: allow .Field access to parenthesized expressions

Change the grammar so that field access is a proper operator.
This introduces a new node, ChainNode, into the public (but
actually internal) API of text/template/parse. For
compatibility, we only use the new node type for the specific
construct, which was not parseable before. Therefore this
should be backward-compatible.

Before, .X.Y was a token in the lexer; this CL breaks it out
into .Y applied to .X. But for compatibility we mush them
back together before delivering. One day we might remove
that hack; it's the simple TODO in parse.go/operand.

This change also provides grammatical distinction between
        f
and
        (f)
which might permit function values later, but not now.

Fixes #3999.

R=golang-dev, dsymonds, gri, rsc, mikesamuel
CC=golang-dev
https://golang.org/cl/6494119

1 files changed, 5 insertions, 2 deletions

diff --git a/src/pkg/text/template/parse/parse_test.go b/src/pkg/text/template/parse/parse_test.go
index 4be4ca077d..0f75c33e77 100644
--- a/src/pkg/text/template/parse/parse_test.go
+++ b/src/pkg/text/template/parse/parse_test.go
@@ -188,6 +188,8 @@ var parseTests = []parseTest{
 		`{{$x := .X | .Y}}`},
 	{"nested pipeline", "{{.X (.Y .Z) (.A | .B .C) (.E)}}", noError,
 		`{{.X (.Y .Z) (.A | .B .C) (.E)}}`},
+	{"field applied to parentheses", "{{(.Y .Z).Field}}", noError,
+		`{{(.Y .Z).Field}}`},
 	{"simple if", "{{if .X}}hello{{end}}", noError,
 		`{{if .X}}"hello"{{end}}`},
 	{"if with else", "{{if .X}}true{{else}}false{{end}}", noError,
@@ -370,8 +372,9 @@ var errorTests = []parseTest{
 		"{{range .X}}",
 		hasError, `unexpected EOF`},
 	{"variable",
-		"{{$a.b := 23}}",
-		hasError, `illegal variable in declaration`},
+		// Declare $x so it's defined, to avoid that error, and then check we don't parse a declaration.
+		"{{$x := 23}}{{with $x.y := 3}}{{$x 23}}{{end}}",
+		hasError, `unexpected ":="`},
 	{"multidecl",
 		"{{$a,$b,$c := 23}}",
 		hasError, `too many declarations`},