text/template: use reflect.Value.FieldByIndexErr

to avoid a crash on a nil pointer as an embedded field.

Fixes #48215

Change-Id: I214faa6e3cf08cdec1c01035e4bbca0900c6a408
Reviewed-on: https://go-review.googlesource.com/c/go/+/357963
Trust: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/text/template/exec.go

@@ -635,10 +635,13 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 	case reflect.Struct:
 		tField, ok := receiver.Type().FieldByName(fieldName)
 		if ok {
-			field := receiver.FieldByIndex(tField.Index)
+			field, err := receiver.FieldByIndexErr(tField.Index)
 			if !tField.IsExported() {
 				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
 			}
+			if err != nil {
+				s.errorf("%v", err)
+			}
 			// If it's a function, we must call it.
 			if hasArgs {
 				s.errorf("%s has arguments but cannot be invoked as function", fieldName)

src/text/template/exec_test.go

@@ -1787,3 +1787,26 @@ func TestIssue39807(t *testing.T) {
 
 	wg.Wait()
 }
+
+// Issue 48215: embedded nil pointer causes panic.
+// Fixed by adding FieldByIndexErr to the reflect package.
+func TestIssue48215(t *testing.T) {
+	type A struct {
+		S string
+	}
+	type B struct {
+		*A
+	}
+	tmpl, err := New("").Parse(`{{ .S }}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tmpl.Execute(io.Discard, B{})
+	// We expect an error, not a panic.
+	if err == nil {
+		t.Fatal("did not get error for nil embedded struct")
+	}
+	if !strings.Contains(err.Error(), "reflect: indirection through nil pointer to embedded struct field A") {
+		t.Fatal(err)
+	}
+}