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

Fixes #11004

common/collections/append.go

@@ -60,13 +60,35 @@ func Append(to any, from ...any) (any, error) {
 		return Slice(from...), nil
 	}
 
+	// See issue #11004.
+	isToASliceOfSlices := tov.Len() > 0
+	for i := 0; i < tov.Len(); i++ {
+		v, isNil := indirect(tov.Index(i))
+		if isNil || v.Kind() != reflect.Slice {
+			isToASliceOfSlices = false
+			break
+		}
+	}
+
+	if isToASliceOfSlices {
+		fv := reflect.ValueOf(from)
+		if !fv.Type().AssignableTo(tot) {
+			// Fall back to a []interface{} slice.
+			tov, _ := indirect(reflect.ValueOf(to))
+			return appendToInterfaceSlice(tov, from)
+		}
+
+		return reflect.Append(tov, fv).Interface(), nil
+	}
+
 	for _, f := range from {
 		fv := reflect.ValueOf(f)
 		if !fv.Type().AssignableTo(tot) {
 			// Fall back to a []interface{} slice.
 			tov, _ := indirect(reflect.ValueOf(to))
 			return appendToInterfaceSlice(tov, from...)
 		}
+
 		tov = reflect.Append(tov, fv)
 	}
 

common/collections/append_test.go

@@ -24,7 +24,7 @@ func TestAppend(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	for _, test := range []struct {
+	for i, test := range []struct {
 		start    any
 		addend   []any
 		expected any
@@ -66,6 +66,40 @@ func TestAppend(t *testing.T) {
 			[]any{&tstSlicer{"a"}},
 			[]any{"a", "b", &tstSlicer{"a"}},
 		},
+		// Issue ##11004
+		{
+			testSlicerInterfaces{testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}}},
+			[]any{testSlicerInterfaces{testSlicerInterfaces{&tstSlicerIn1{"c"}}}},
+			testSlicerInterfaces{
+				testSlicerInterfaces{
+					&tstSlicerIn1{TheName: "a"},
+					&tstSlicerIn1{TheName: "b"},
+				},
+				testSlicerInterfaces{
+					&tstSlicerIn1{TheName: "c"},
+				},
+			},
+		},
+		{
+			[]any{[]string{"a", "b"}},
+			[]any{&tstSlicer{"a"}},
+			[]interface{}{
+				[]string{"a", "b"},
+				[]interface{}{
+					&tstSlicer{TheName: "a"},
+				},
+			},
+		},
+		{
+			[]any{[]string{"a", "b"}},
+			[]any{"c"},
+			[]interface{}{
+				[]string{"a", "b"},
+				[]interface{}{
+					"c",
+				},
+			},
+		},
 		// Errors
 		{"", []any{[]string{"a", "b"}}, false},
 		// No string concatenation.
@@ -85,6 +119,6 @@ func TestAppend(t *testing.T) {
 		}
 
 		c.Assert(err, qt.IsNil)
-		c.Assert(result, qt.DeepEquals, test.expected)
+		c.Assert(result, qt.DeepEquals, test.expected, qt.Commentf("[%d] %v", i, test.start))
 	}
 }

common/collections/slice_test.go

@@ -34,6 +34,10 @@ type testSlicerInterface interface {
 
 type testSlicerInterfaces []testSlicerInterface
 
+func (t testSlicerInterfaces) Name() string {
+	return "testSlicerInterfaces"
+}
+
 type tstSlicerIn1 struct {
 	TheName string
 }

tpl/collections/append_test.go

@@ -34,6 +34,11 @@ func TestAppend(t *testing.T) {
 		{[]string{"a", "b"}, []any{"c"}, []string{"a", "b", "c"}},
 		{[]string{"a", "b"}, []any{"c", "d", "e"}, []string{"a", "b", "c", "d", "e"}},
 		{[]string{"a", "b"}, []any{[]string{"c", "d", "e"}}, []string{"a", "b", "c", "d", "e"}},
+		// Issue #11004
+		{[]any{[]any{"a"}}, []any{"b"}, []any{[]any{"a"}, []any{"b"}}},
+		{[]any{[]string{"a"}}, []any{"b"}, []any{[]string{"a"}, []any{"b"}}},
+		{[]any{[]any{"a"}, "b"}, []any{"c"}, []any{[]any{"a"}, "b", "c"}},
+
 		// Errors
 		{"", []any{[]string{"a", "b"}}, false},
 		{[]string{"a", "b"}, []any{}, false},