text/template: pipelined arg was not typechecked

Without this fix, an erroneous template causes a panic; should be caught safely.
The bug did not affect correct templates.

Fixes #3267.

R=golang-dev, dsymonds, rsc
CC=golang-dev
https://golang.org/cl/5900065

src/pkg/text/template/exec.go

@@ -491,7 +491,11 @@ func (s *state) evalCall(dot, fun reflect.Value, name string, args []parse.Node,
 	}
 	// Add final value if necessary.
 	if final.IsValid() {
-		argv[i] = final
+		t := typ.In(typ.NumIn() - 1)
+		if typ.IsVariadic() {
+			t = t.Elem()
+		}
+		argv[i] = s.validateType(final, t)
 	}
 	result := fun.Call(argv)
 	// If we have an error that is not nil, stop execution and return that error to the caller.
@@ -507,6 +511,7 @@ func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Valu
 		switch typ.Kind() {
 		case reflect.Interface, reflect.Ptr, reflect.Chan, reflect.Map, reflect.Slice, reflect.Func:
 			// An untyped nil interface{}. Accept as a proper nil value.
+			// TODO: Can we delete the other types in this list? Should we?
 			value = reflect.Zero(typ)
 		default:
 			s.errorf("invalid value; expected %s", typ)

src/pkg/text/template/exec_test.go

@@ -470,6 +470,9 @@ var execTests = []execTest{
 	{"bug7a", "{{3 2}}", "", tVal, false},
 	{"bug7b", "{{$x := 1}}{{$x 2}}", "", tVal, false},
 	{"bug7c", "{{$x := 1}}{{3 | $x}}", "", tVal, false},
+	// Pipelined arg was not being type-checked.
+	{"bug8a", "{{3|oneArg}}", "", tVal, false},
+	{"bug8b", "{{4|dddArg 3}}", "", tVal, false},
 }
 
 func zeroArgs() string {
@@ -480,6 +483,10 @@ func oneArg(a string) string {
 	return "oneArg=" + a
 }
 
+func dddArg(a int, b ...string) string {
+	return fmt.Sprintln(a, b)
+}
+
 // count returns a channel that will deliver n sequential 1-letter strings starting at "a"
 func count(n int) chan string {
 	if n == 0 {
@@ -504,6 +511,7 @@ func testExecute(execTests []execTest, template *Template, t *testing.T) {
 	b := new(bytes.Buffer)
 	funcs := FuncMap{
 		"count":    count,
+		"dddArg":   dddArg,
 		"oneArg":   oneArg,
 		"typeOf":   typeOf,
 		"vfunc":    vfunc,