handle nils correctly in ParamExpander

- previously, for types that can be nil (specifically slice, map;
  interface and pointer are correct), a new value of backing type
  was created instead of making a new nil
- this is incorrect because it affects the structure of the object; more
  specifically the failure observed was with a struct having a field of
  type `[]string`. Instead of keeping a `[]string(nil)` here, this would
  transform into an empty slice `[]string{}`

Signed-off-by: mimir-d <[email protected]>

Author: mimir-d

pkg/test/param_expander.go

@@ -74,6 +74,12 @@ func (pe *ParamExpander) ExpandObject(obj interface{}, out interface{}) error {
 			vout.Set(val)
 
 		case reflect.Map:
+			if vin.IsNil() {
+				// handle nil maps, eg. map[T]U(nil)
+				vout.Set(vin)
+				break
+			}
+
 			val := reflect.MakeMap(vin.Type())
 			iter := vin.MapRange()
 			for iter.Next() {
@@ -112,6 +118,12 @@ func (pe *ParamExpander) ExpandObject(obj interface{}, out interface{}) error {
 			vout.Set(val.Elem())
 
 		case reflect.Slice:
+			if vin.IsNil() {
+				// handle nil slices, eg. []T(nil)
+				vout.Set(vin)
+				break
+			}
+
 			elemType := vin.Type().Elem()
 			len := vin.Len()
 

pkg/test/param_expander_test.go

@@ -165,12 +165,18 @@ func TestParamExpander(t *testing.T) {
 
 	t.Run("nulls", func(t *testing.T) {
 		type testObj struct {
-			Str  *string
-			Intf any
+			Str    *string
+			Intf   any
+			Ints   []int
+			IntMap map[int]int
 		}
 
 		// both fields null
 		input := &testObj{}
+		require.Nil(t, input.Str)
+		require.Nil(t, input.Intf)
+		require.Nil(t, input.Ints)
+		require.Nil(t, input.IntMap)
 
 		e := NewParamExpander(target, &mockVars{})
 
@@ -180,5 +186,7 @@ func TestParamExpander(t *testing.T) {
 
 		require.Nil(t, out.Str)
 		require.Nil(t, out.Intf)
+		require.Nil(t, out.Ints)
+		require.Nil(t, out.IntMap)
 	})
 }