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, 17 insertions, 2 deletions

diff --git a/src/pkg/text/template/exec.go b/src/pkg/text/template/exec.go
index 1739a86179..5e127d7db4 100644
--- a/src/pkg/text/template/exec.go
+++ b/src/pkg/text/template/exec.go
@@ -315,9 +315,15 @@ func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final ref
 	switch n := firstWord.(type) {
 	case *parse.FieldNode:
 		return s.evalFieldNode(dot, n, cmd.Args, final)
+	case *parse.ChainNode:
+		return s.evalChainNode(dot, n, cmd.Args, final)
 	case *parse.IdentifierNode:
 		// Must be a function.
 		return s.evalFunction(dot, n.Ident, cmd.Args, final)
+	case *parse.PipeNode:
+		// Parenthesized pipeline. The arguments are all inside the pipeline; final is ignored.
+		// TODO: is this right?
+		return s.evalPipeline(dot, n)
 	case *parse.VariableNode:
 		return s.evalVariableNode(dot, n, cmd.Args, final)
 	}
@@ -367,6 +373,15 @@ func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []
 	return s.evalFieldChain(dot, dot, field.Ident, args, final)
 }
 
+func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
+	// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
+	pipe := s.evalArg(dot, nil, chain.Node)
+	if len(chain.Field) == 0 {
+		s.errorf("internal error: no fields in evalChainNode")
+	}
+	return s.evalFieldChain(dot, pipe, chain.Field, args, final)
+}
+
 func (s *state) evalVariableNode(dot reflect.Value, v *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
 	// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
 	value := s.varValue(v.Ident[0])
@@ -521,13 +536,13 @@ func canBeNil(typ reflect.Type) bool {
 // validateType guarantees that the value is valid and assignable to the type.
 func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
 	if !value.IsValid() {
-		if canBeNil(typ) {
+		if typ == nil || canBeNil(typ) {
 			// An untyped nil interface{}. Accept as a proper nil value.
 			return reflect.Zero(typ)
 		}
 		s.errorf("invalid value; expected %s", typ)
 	}
-	if !value.Type().AssignableTo(typ) {
+	if typ != nil && !value.Type().AssignableTo(typ) {
 		if value.Kind() == reflect.Interface && !value.IsNil() {
 			value = value.Elem()
 			if value.Type().AssignableTo(typ) {