text/template: support range-over-func

For #66107

Change-Id: I2fcd04bebe80346dbd244ab7ea09cbe6010b9d8e
GitHub-Last-Rev: 5ebf615db5889a04738c555c651e07c1fd287748
GitHub-Pull-Request: golang/go#68329
Reviewed-on: https://go-review.googlesource.com/c/go/+/596956
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 5b35b3e5a8..96d2f50ef8 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -434,6 +434,43 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 		return
 	case reflect.Invalid:
 		break // An invalid value is likely a nil map, etc. and acts like an empty map.
+	case reflect.Func:
+		if val.Type().CanSeq() {
+			if len(r.Pipe.Decl) > 1 {
+				s.errorf("can't use %s iterate over more than one variable", val)
+				break
+			}
+			run := false
+			for v := range val.Seq() {
+				run = true
+				// Pass element as second value,
+				// as we do for channels.
+				oneIteration(reflect.Value{}, v)
+			}
+			if !run {
+				break
+			}
+			return
+		}
+		if val.Type().CanSeq2() {
+			run := false
+			for i, v := range val.Seq2() {
+				run = true
+				if len(r.Pipe.Decl) > 1 {
+					oneIteration(i, v)
+				} else {
+					// If there is only one range variable,
+					// oneIteration will use the
+					// second value.
+					oneIteration(reflect.Value{}, i)
+				}
+			}
+			if !run {
+				break
+			}
+			return
+		}
+		fallthrough
 	default:
 		s.errorf("range can't iterate over %v", val)
 	}