common/collections: Allow a mix of slice types in append/Scratch.Add

The type handling in these was improved in Hugo 0.49, but this also meant that it was no longer possible to start out with a string slice and later append `Page` etc. to it.

This commit makes sure that the old behaviour is now possible again by falling back to a `[]interface{}` as a last resort.

Fixes #5361

common/collections/append.go

@@ -48,6 +48,10 @@ func Append(to interface{}, from ...interface{}) (interface{}, error) {
 				// 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)
+
 				}
 			}
 		}
@@ -60,7 +64,8 @@ func Append(to interface{}, from ...interface{}) (interface{}, error) {
 	for _, f := range from {
 		fv := reflect.ValueOf(f)
 		if !fv.Type().AssignableTo(tot) {
-			return nil, fmt.Errorf("append element type mismatch: expected %v, got %v", tot, fv.Type())
+			// Fall back to a []interface{} slice.
+			return appendToInterfaceSlice(tov, from...)
 		}
 		tov = reflect.Append(tov, fv)
 	}
@@ -68,6 +73,32 @@ func Append(to interface{}, from ...interface{}) (interface{}, error) {
 	return tov.Interface(), nil
 }
 
+func appendToInterfaceSliceFromValues(slice1, slice2 reflect.Value) ([]interface{}, error) {
+	var tos []interface{}
+
+	for _, slice := range []reflect.Value{slice1, slice2} {
+		for i := 0; i < slice.Len(); i++ {
+			tos = append(tos, slice.Index(i).Interface())
+		}
+	}
+
+	return tos, nil
+}
+
+func appendToInterfaceSlice(tov reflect.Value, from ...interface{}) ([]interface{}, error) {
+	var tos []interface{}
+
+	for i := 0; i < tov.Len(); i++ {
+		tos = append(tos, tov.Index(i).Interface())
+	}
+
+	for _, v := range from {
+		tos = append(tos, v)
+	}
+
+	return tos, nil
+}
+
 // indirect is borrowed from the Go stdlib: 'text/template/exec.go'
 // TODO(bep) consolidate
 func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {