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/21705
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index e507e917fe..bc2aa683ec 100644
--- a/src/text/template/exec_test.go
+++ b/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)
+	}
+}