From 5c7fad23a3f8631e10264aa42efafe98dbf853d5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B8rn=20Erik=20Pedersen?=
 <bjorn.erik.pedersen@gmail.com>
Date: Wed, 31 Dec 2025 11:10:19 +0100
Subject: [PATCH] tpl/collections: Fix apply to work with built-in funcs like
 len

Fixes #13418
---
 tpl/collections/apply.go                        |  8 +++++++-
 tpl/collections/collections_integration_test.go | 15 +++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/tpl/collections/apply.go b/tpl/collections/apply.go
index 5b3cbe54d14..23c6a74976a 100644
--- a/tpl/collections/apply.go
+++ b/tpl/collections/apply.go
@@ -64,6 +64,8 @@ func (ns *Namespace) Apply(ctx context.Context, c any, fname string, args ...any
 	}
 }
 
+var typeOfReflectValue = reflect.TypeOf(reflect.Value{})
+
 func applyFnToThis(ctx context.Context, fn, this reflect.Value, args ...any) (reflect.Value, error) {
 	num := fn.Type().NumIn()
 	if num > 0 && hreflect.IsContextType(fn.Type().In(0)) {
@@ -91,7 +93,11 @@ func applyFnToThis(ctx context.Context, fn, this reflect.Value, args ...any) (re
 	}*/
 
 	for i := range num {
-		// AssignableTo reports whether xt is assignable to type targ.
+		// Go's built-in template funcs (e.g. len) use reflect.Value as argument type.
+		if fn.Type().In(i) == typeOfReflectValue && n[i].Type() != typeOfReflectValue {
+			n[i] = reflect.ValueOf(n[i])
+		}
+
 		if xt, targ := n[i].Type(), fn.Type().In(i); !xt.AssignableTo(targ) {
 			return reflect.ValueOf(nil), errors.New("called apply using " + xt.String() + " as type " + targ.String())
 		}
diff --git a/tpl/collections/collections_integration_test.go b/tpl/collections/collections_integration_test.go
index 1adfd5abf5c..a1a14852419 100644
--- a/tpl/collections/collections_integration_test.go
+++ b/tpl/collections/collections_integration_test.go
@@ -39,6 +39,21 @@ baseURL = 'http://example.com/'
 `)
 }
 
+func TestApplyBuiltInIssue13418(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+baseURL = 'http://example.com/'
+-- layouts/home.html --
+len: {{ apply (slice "hello") "len" "." }}
+not: {{ apply (slice "hello") "not" "." }}
+`
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/index.html", "len: [5]", "not: [false]")
+}
+
 // Issue 9865
 func TestSortStable(t *testing.T) {
 	t.Parallel()