text/template: emit field error over nil pointer error where appropriate

When evaluating "{{.MissingField}}" on a nil *T, Exec returns
"can't evaluate field MissingField in type *T" instead of
"nil pointer evaluating *T.MissingField".

Fixes golang/go#15125

Change-Id: I6e73f61b8a72c694179c1f8cdc808766c90b6f57
Reviewed-on: https://go-review.googlesource.com/21705Reviewed-by: Rob Pike <r@golang.org>

src/text/template/exec.go

@@ -538,14 +538,14 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 		return s.evalCall(dot, method, node, fieldName, args, final)
 	}
 	hasArgs := len(args) > 1 || final.IsValid()
-	// It's not a method; must be a field of a struct or an element of a map. The receiver must not be nil.
-	if isNil {
-		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
-	}
+	// It's not a method; must be a field of a struct or an element of a map.
 	switch receiver.Kind() {
 	case reflect.Struct:
 		tField, ok := receiver.Type().FieldByName(fieldName)
 		if ok {
+			if isNil {
+				s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
+			}
 			field := receiver.FieldByIndex(tField.Index)
 			if tField.PkgPath != "" { // field is unexported
 				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
@@ -556,8 +556,10 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 			}
 			return field
 		}
-		s.errorf("%s is not a field of struct type %s", fieldName, typ)
 	case reflect.Map:
+		if isNil {
+			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
+		}
 		// If it's a map, attempt to use the field name as a key.
 		nameVal := reflect.ValueOf(fieldName)
 		if nameVal.Type().AssignableTo(receiver.Type().Key()) {

src/text/template/exec_test.go

@@ -1280,3 +1280,20 @@ func TestBlock(t *testing.T) {
 		t.Errorf("got %q, want %q", got, want2)
 	}
 }
+
+// Check that calling an invalid field on nil pointer prints
+// a field error instead of a distracting nil pointer error.
+// https://golang.org/issue/15125
+func TestMissingFieldOnNil(t *testing.T) {
+	tmpl := Must(New("tmpl").Parse("{{.MissingField}}"))
+	var d *T
+	err := tmpl.Execute(ioutil.Discard, d)
+	got := "<nil>"
+	if err != nil {
+		got = err.Error()
+	}
+	want := "can't evaluate field MissingField in type *template.T"
+	if !strings.HasSuffix(got, want) {
+		t.Errorf("got error %q, want %q", got, want)
+	}
+}