text/template: copy Decl field when copying PipeNode

Fixes #24791

Change-Id: I62ac17313e6e09796586911d88191a36d67f9aa1
Reviewed-on: https://go-review.googlesource.com/106115
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/html/template/clone_test.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"strings"
 	"sync"
 	"testing"
 	"text/template/parse"
@@ -262,3 +263,17 @@ func TestCloneRedefinedName(t *testing.T) {
 		}
 	}
 }
+
+// Issue 24791.
+func TestClonePipe(t *testing.T) {
+	a := Must(New("a").Parse(`{{define "a"}}{{range $v := .A}}{{$v}}{{end}}{{end}}`))
+	data := struct{ A []string }{A: []string{"hi"}}
+	b := Must(a.Clone())
+	var buf strings.Builder
+	if err := b.Execute(&buf, &data); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := buf.String(), "hi"; got != want {
+		t.Errorf("got %q want %q", got, want)
+	}
+}

src/text/template/parse/node.go

@@ -192,6 +192,7 @@ func (p *PipeNode) CopyPipe() *PipeNode {
 		vars = append(vars, d.Copy().(*AssignNode))
 	}
 	n := p.tr.newPipeline(p.Pos, p.Line, vars)
+	n.Decl = p.Decl
 	for _, c := range p.Cmds {
 		n.append(c.Copy().(*CommandNode))
 	}