tpl/collections: Fix append when appending a slice to a slice of slices

Fixes gohugoio#11093

common/collections/append.go

@@ -22,6 +22,9 @@ import (
 // If length of from is one and the only element is a slice of same type as to,
 // it will be appended.
 func Append(to any, from ...any) (any, error) {
+	if len(from) == 0 {
+		return to, nil
+	}
 	tov, toIsNil := indirect(reflect.ValueOf(to))
 
 	toIsNil = toIsNil || to == nil
@@ -32,26 +35,40 @@ func Append(to any, from ...any) (any, error) {
 			return nil, fmt.Errorf("expected a slice, got %T", to)
 		}
 
+		fromv := reflect.ValueOf(from[0])
+		fromt := fromv.Type()
+		if fromt.Kind() == reflect.Slice {
+			fromt = fromt.Elem()
+		}
+
 		tot = tov.Type().Elem()
+		if tot.Kind() == reflect.Slice {
+			totvt := tot.Elem()
+			if totvt != fromt {
+				return nil, fmt.Errorf("cannot append slice of %s to slice of %s", fromt, totvt)
+			} else {
+				return reflect.Append(tov, fromv).Interface(), nil
+			}
+		}
+
 		toIsNil = tov.Len() == 0
 
 		if len(from) == 1 {
-			fromv := reflect.ValueOf(from[0])
+
 			if fromv.Kind() == reflect.Slice {
 				if toIsNil {
 					// If we get nil []string, we just return the []string
 					return from[0], nil
 				}
 
-				fromt := reflect.TypeOf(from[0]).Elem()
-
 				// If we get []string []string, we append the from slice to to
 				if tot == fromt {
 					return reflect.AppendSlice(tov, fromv).Interface(), nil
 				} else if !fromt.AssignableTo(tot) {
 					// Fall back to a []interface{} slice.
 					return appendToInterfaceSliceFromValues(tov, fromv)
 				}
+
 			}
 		}
 	}

common/collections/append_test.go

@@ -88,3 +88,36 @@ func TestAppend(t *testing.T) {
 		c.Assert(result, qt.DeepEquals, test.expected)
 	}
 }
+
+// #11093
+func TestAppendToMultiDimensionalSlice(t *testing.T) {
+	t.Parallel()
+	c := qt.New(t)
+
+	for _, test := range []struct {
+		to       any
+		from     any
+		expected any
+	}{
+		{[][]string{{"a", "b"}},
+			[]string{"c", "d"},
+			[][]string{
+				{"a", "b"},
+				{"c", "d"},
+			},
+		},
+		{[][]string{{"a", "b"}},
+			[]int{1, 2},
+			false,
+		},
+	} {
+		result, err := Append(test.to, test.from)
+		if b, ok := test.expected.(bool); ok && !b {
+			c.Assert(err, qt.Not(qt.IsNil))
+		} else {
+			c.Assert(err, qt.IsNil)
+			c.Assert(result, qt.DeepEquals, test.expected)
+		}
+	}
+
+}

tpl/collections/integration_test.go

@@ -73,3 +73,34 @@ Desc: [map[a:3 b:3] map[a:3 b:1] map[a:3 b:1] map[a:3 b:1] map[a:3 b:0] map[a:3
 
 	}
 }
+
+// Issue #11004.
+func TestAppendSliceToASliceOfSlices(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/index.html --
+{{ $obj := slice (slice "a") }}
+{{ $obj = $obj | append (slice "b") }}
+{{ $obj = $obj | append (slice "c") }}
+
+{{ $obj }}
+
+
+  `
+
+	for i := 0; i < 4; i++ {
+
+		b := hugolib.NewIntegrationTestBuilder(
+			hugolib.IntegrationTestConfig{
+				T:           t,
+				TxtarString: files,
+			},
+		).Build()
+
+		b.AssertFileContent("public/index.html", "[[a] [b] [c]]")
+
+	}
+
+}