text/template: add support for reflect.Value args, results in funcs

Add support for passing reflect.Values to and returning reflect.Values from
any registered functions in the FuncMap, much as if they were
interface{} values. Keeping the reflect.Value instead of round-tripping
to interface{} preserves addressability of the value, which is important
for method lookup.

Change index and a few other built-in functions to use reflect.Values,
making a loop using explicit indexing now match the semantics that
range has always had.

Fixes #14916.

Change-Id: Iae1a2fd9bb426886a7fcd9204f30a2d6ad4646ad
Reviewed-on: https://go-review.googlesource.com/31462
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index 3ef065edcf..7092961850 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -1310,3 +1310,26 @@ func TestMaxExecDepth(t *testing.T) {
 		t.Errorf("got error %q; want %q", got, want)
 	}
 }
+
+func TestAddrOfIndex(t *testing.T) {
+	// golang.org/issue/14916.
+	// Before index worked on reflect.Values, the .String could not be
+	// found on the (incorrectly unaddressable) V value,
+	// in contrast to range, which worked fine.
+	// Also testing that passing a reflect.Value to tmpl.Execute works.
+	texts := []string{
+		`{{range .}}{{.String}}{{end}}`,
+		`{{with index . 0}}{{.String}}{{end}}`,
+	}
+	for _, text := range texts {
+		tmpl := Must(New("tmpl").Parse(text))
+		var buf bytes.Buffer
+		err := tmpl.Execute(&buf, reflect.ValueOf([]V{{1}}))
+		if err != nil {
+			t.Fatal("%s: Execute: %v", text, err)
+		}
+		if buf.String() != "<1>" {
+			t.Fatalf("%s: template output = %q, want %q", text, buf, "<1>")
+		}
+	}
+}