text/template: handle option missingkey=error consistently

The existing implementation of text/template handles the option
"missingkey=error" in an inconsitent manner:  If the provided data is
a nil-interface, no error is returned (despite the fact that no key
can be found in it).

This patch makes text/template return an error if "missingkey=error"
is set and the provided data is a not a valid reflect.Value.

Fixes #15356

Change-Id: Ia0a83da48652ecfaf31f18bdbd78cb21dbca1164
Reviewed-on: https://go-review.googlesource.com/31638Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/text/template/exec.go

@@ -541,6 +541,9 @@ func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd
 // value of the pipeline, if any.
 func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
 	if !receiver.IsValid() {
+		if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
+			s.errorf("nil data; no entry for key %q", fieldName)
+		}
 		return zero
 	}
 	typ := receiver.Type()

src/text/template/exec_test.go

@@ -1142,6 +1142,12 @@ func TestMissingMapKey(t *testing.T) {
 	if err == nil {
 		t.Errorf("expected error; got none")
 	}
+	// same Option, but now a nil interface: ask for an error
+	err = tmpl.Execute(&b, nil)
+	t.Log(err)
+	if err == nil {
+		t.Errorf("expected error for nil-interface; got none")
+	}
 }
 
 // Test that the error message for multiline unterminated string