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

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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/21705
Reviewed-by: Rob Pike <r@golang.org>
This commit is contained in:
Andrew Gerrand 2016-04-08 15:39:32 +10:00
parent 0004f34cef
commit 0fd270ab7a
2 changed files with 24 additions and 5 deletions
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12
src/text/template/exec.go
View File

@ -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()) {

17
src/text/template/exec_test.go
View File

@ -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)
}
}