text/template: check for literals in chain of terms

The current parser ignores obvious errors such as:
{{0.1.E}}
{{true.any}}
{{"hello".wrong}}
{{nil.E}}

The common problem is that a chain is built from
a literal value. It then panics at execution time.

Furthermore, a double dot triggers the same behavior:
{{..E}}

Addresses a TODO left in Tree.operand to catch these
errors at parsing time.

Note that identifiers can include a '.', and pipelines
could return an object which a field can be derived
from (like a variable), so they are excluded from the check.

Fixes #10615

Change-Id: I903706d1c17861b5a8354632c291e73c9c0bc4e1
Reviewed-on: https://go-review.googlesource.com/9621Reviewed-by: Rob Pike <r@golang.org>

src/text/template/exec_test.go

@@ -527,8 +527,10 @@ var execTests = []execTest{
 	{"bug12XE", "{{printf `%T` 0XEE}}", "int", T{}, true},
 	// Chained nodes did not work as arguments. Issue 8473.
 	{"bug13", "{{print (.Copy).I}}", "17", tVal, true},
-	// Didn't protect against explicit nil in field chains.
-	{"bug14", "{{nil.True}}", "", tVal, false},
+	// Didn't protect against nil or literal values in field chains.
+	{"bug14a", "{{(nil).True}}", "", tVal, false},
+	{"bug14b", "{{$x := nil}}{{$x.anything}}", "", tVal, false},
+	{"bug14c", `{{$x := (1.0)}}{{$y := ("hello")}}{{$x.anything}}{{$y.true}}`, "", tVal, false},
 }
 
 func zeroArgs() string {

src/text/template/parse/parse.go

@@ -554,7 +554,7 @@ func (t *Tree) command() *CommandNode {
 			t.backup()
 		case itemPipe:
 		default:
-			t.errorf("unexpected %s in operand; missing space?", token)
+			t.errorf("unexpected %s in operand", token)
 		}
 		break
 	}
@@ -582,12 +582,15 @@ func (t *Tree) operand() Node {
 		// Compatibility with original API: If the term is of type NodeField
 		// or NodeVariable, just put more fields on the original.
 		// Otherwise, keep the Chain node.
-		// TODO: Switch to Chains always when we can.
+		// Obvious parsing errors involving literal values are detected here.
+		// More complex error cases will have to be handled at execution time.
 		switch node.Type() {
 		case NodeField:
 			node = t.newField(chain.Position(), chain.String())
 		case NodeVariable:
 			node = t.newVariable(chain.Position(), chain.String())
+		case NodeBool, NodeString, NodeNumber, NodeNil, NodeDot:
+			t.errorf("unexpected . after term %q", node.String())
 		default:
 			node = chain
 		}

src/text/template/parse/parse_test.go

@@ -260,6 +260,14 @@ var parseTests = []parseTest{
 	{"bug1a", "{{$x:=.}}{{$x!2}}", hasError, ""},                     // ! is just illegal here.
 	{"bug1b", "{{$x:=.}}{{$x+2}}", hasError, ""},                     // $x+2 should not parse as ($x) (+2).
 	{"bug1c", "{{$x:=.}}{{$x +2}}", noError, "{{$x := .}}{{$x +2}}"}, // It's OK with a space.
+	// dot following a literal value
+	{"dot after integer", "{{1.E}}", hasError, ""},
+	{"dot after float", "{{0.1.E}}", hasError, ""},
+	{"dot after boolean", "{{true.E}}", hasError, ""},
+	{"dot after char", "{{'a'.any}}", hasError, ""},
+	{"dot after string", `{{"hello".guys}}`, hasError, ""},
+	{"dot after dot", "{{..E}}", hasError, ""},
+	{"dot after nil", "{{nil.E}}", hasError, ""},
 }
 
 var builtins = map[string]interface{}{
@@ -378,7 +386,7 @@ var errorTests = []parseTest{
 		hasError, `unexpected ")"`},
 	{"space",
 		"{{`x`3}}",
-		hasError, `missing space?`},
+		hasError, `in operand`},
 	{"idchar",
 		"{{a#}}",
 		hasError, `'#'`},
@@ -410,6 +418,9 @@ var errorTests = []parseTest{
 	{"undefvar",
 		"{{$a}}",
 		hasError, `undefined variable`},
+	{"wrongdot",
+		"{{true.any}}",
+		hasError, `unexpected . after term`},
 }
 
 func TestErrors(t *testing.T) {