text/template: never call reflect.Zero(nil)

It makes no sense to try to get the zero value of a nil type, hence the
panic. When we have a nil type, use reflect.ValueOf(nil) instead.

This was showing itself if one used a missing field on the data between
parentheses, when the data was a nil interface:

	t := template.Must(template.New("test").Parse(`{{ (.).foo }}`))
	var v interface{}
	t.Execute(os.Stdout, v)

Resulting in:

	panic: reflect: Zero(nil) [recovered]
		panic: reflect: Zero(nil)

Fixes #21171.

Change-Id: Ifcc4a0c67e6df425b65bc9f82fde6fcf03828579
Reviewed-on: https://go-review.googlesource.com/84482
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/text/template/exec.go

@@ -691,8 +691,12 @@ 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 typ == nil || canBeNil(typ) {
+		if typ == nil {
 			// An untyped nil interface{}. Accept as a proper nil value.
+			return reflect.ValueOf(nil)
+		}
+		if canBeNil(typ) {
+			// Like above, but use the zero value of the non-nil type.
 			return reflect.Zero(typ)
 		}
 		s.errorf("invalid value; expected %s", typ)

src/text/template/exec_test.go

@@ -330,6 +330,10 @@ var execTests = []execTest{
 	{"empty with struct", "{{.Empty4}}", "{UinEmpty}", tVal, true},
 	{"empty with struct, field", "{{.Empty4.V}}", "UinEmpty", tVal, true},
 
+	// Edge cases with <no value> with an interface value
+	{"field on interface", "{{.foo}}", "<no value>", nil, true},
+	{"field on parenthesized interface", "{{(.).foo}}", "<no value>", nil, true},
+
 	// Method calls.
 	{".Method0", "-{{.Method0}}-", "-M0-", tVal, true},
 	{".Method1(1234)", "-{{.Method1 1234}}-", "-1234-", tVal, true},