2 files changed, 22 insertions, 4 deletions
diff --git a/src/html/template/escape_test.go b/src/html/template/escape_test.go
index f5a4ce1736..92f12ca0e0 100644
--- a/src/html/template/escape_test.go
+++ b/src/html/template/escape_test.go
@@ -1840,7 +1840,7 @@ func TestErrorOnUndefined(t *testing.T) {
 
 	err := tmpl.Execute(nil, nil)
 	if err == nil {
-		t.Error("expected error")
+		t.Fatal("expected error")
 	}
 	if !strings.Contains(err.Error(), "incomplete") {
 		t.Errorf("expected error about incomplete template; got %s", err)
@@ -1860,10 +1860,10 @@ func TestIdempotentExecute(t *testing.T) {
 	for i := 0; i < 2; i++ {
 		err = tmpl.ExecuteTemplate(got, "hello", nil)
 		if err != nil {
-			t.Errorf("unexpected error: %s", err)
+			t.Fatalf("unexpected error: %s", err)
 		}
 		if got.String() != want {
-			t.Errorf("after executing template \"hello\", got:\n\t%q\nwant:\n\t%q\n", got.String(), want)
+			t.Fatalf("after executing template \"hello\", got:\n\t%q\nwant:\n\t%q\n", got.String(), want)
 		}
 		got.Reset()
 	}
@@ -1871,7 +1871,7 @@ func TestIdempotentExecute(t *testing.T) {
 	// "main" does not cause the output of "hello" to change.
 	err = tmpl.ExecuteTemplate(got, "main", nil)
 	if err != nil {
-		t.Errorf("unexpected error: %s", err)
+		t.Fatalf("unexpected error: %s", err)
 	}
 	// If the HTML escaper is added again to the action {{"Ladies & Gentlemen!"}},
 	// we would expected to see the ampersand overescaped to "&amp;amp;".
@@ -1881,6 +1881,19 @@ func TestIdempotentExecute(t *testing.T) {
 	}
 }
 
+// This covers issue #21844.
+func TestAddExistingTreeError(t *testing.T) {
+	tmpl := Must(New("foo").Parse(`<p>{{.}}</p>`))
+	tmpl, err := tmpl.AddParseTree("bar", tmpl.Tree)
+	if err == nil {
+		t.Fatalf("expected error after AddParseTree")
+	}
+	const want = `html/template: cannot add parse tree that template "foo" already references`
+	if got := err.Error(); got != want {
+		t.Errorf("got error:\n\t%q\nwant:\n\t%q\n", got, want)
+	}
+}
+
 func BenchmarkEscapedExecute(b *testing.B) {
 	tmpl := Must(New("t").Parse(`<a onclick="alert('{{.}}')">{{.}}</a>`))
 	var buf bytes.Buffer
diff --git a/src/html/template/template.go b/src/html/template/template.go
index 6a661bf6e5..d77aa3d7df 100644
--- a/src/html/template/template.go
+++ b/src/html/template/template.go
@@ -219,6 +219,11 @@ func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error
 
 	t.nameSpace.mu.Lock()
 	defer t.nameSpace.mu.Unlock()
+	for _, tmpl := range t.set {
+		if tmpl.Tree == tree {
+			return nil, fmt.Errorf("html/template: cannot add parse tree that template %q already references", tmpl.Name())
+		}
+	}
 	text, err := t.text.AddParseTree(name, tree)
 	if err != nil {
 		return nil, err