Fix bug where CEL listOfString.join() results in unexpected error

staging/src/k8s.io/apiextensions-apiserver/pkg/apiserver/schema/cel/validation_test.go

@@ -338,6 +338,21 @@ func TestValidationExpressions(t *testing.T) {
 				"self.val1 + self.val2 == [1, 2, 3, 1, 2, 3]",
 				"self.val1 + [4, 5] == [1, 2, 3, 4, 5]",
 			},
+			errors: map[string]string{
+				// Mixed type lists are not allowed since we have HomogeneousAggregateLiterals enabled
+				"[1, 'a', false].filter(x, string(x) == 'a')": "expected type 'int' but found 'string'",
+			},
+		},
+		{name: "string lists",
+			obj:    objs([]interface{}{"a", "b", "c"}),
+			schema: schemas(listType(&stringType)),
+			valid: []string{
+				// Join function
+				"self.val1.join('-') == 'a-b-c'",
+				"['a', 'b', 'c'].join('-') == 'a-b-c'",
+				"self.val1.join() == 'abc'",
+				"['a', 'b', 'c'].join() == 'abc'",
+			},
 		},
 		{name: "listSets",
 			obj:    objs([]interface{}{"a", "b", "c"}, []interface{}{"a", "c", "b"}),
@@ -351,6 +366,12 @@ func TestValidationExpressions(t *testing.T) {
 				"!('x' in self.val1)",
 				"self.val1 + self.val2 == ['a', 'b', 'c']",
 				"self.val1 + ['c', 'd'] == ['a', 'b', 'c', 'd']",
+
+				// Join function
+				"self.val1.join('-') == 'a-b-c'",
+				"['a', 'b', 'c'].join('-') == 'a-b-c'",
+				"self.val1.join() == 'abc'",
+				"['a', 'b', 'c'].join() == 'abc'",
 			},
 		},
 		{name: "listMaps",
@@ -403,6 +424,10 @@ func TestValidationExpressions(t *testing.T) {
 				"!('k3' in self.val1)",
 				"self.val1 == {'k1': 'a', 'k2': 'b'}",
 			},
+			errors: map[string]string{
+				// Mixed type maps are not allowed since we have HomogeneousAggregateLiterals enabled
+				"{'k1': 'a', 'k2': 1, 'k2': false}": "expected type 'string' but found 'int'",
+			},
 		},
 		{name: "objects",
 			obj: map[string]interface{}{

staging/src/k8s.io/apiserver/pkg/cel/common/values.go

@@ -26,9 +26,10 @@ import (
 	"github.com/google/cel-go/common/types/ref"
 	"github.com/google/cel-go/common/types/traits"
 
+	"k8s.io/kube-openapi/pkg/validation/strfmt"
+
 	"k8s.io/apimachinery/pkg/api/equality"
 	"k8s.io/apiserver/pkg/cel"
-	"k8s.io/kube-openapi/pkg/validation/strfmt"
 )
 
 // UnstructuredToVal converts a Kubernetes unstructured data element to a CEL Val.
@@ -425,7 +426,22 @@ var _ = traits.Lister(&unstructuredList{})
 func (t *unstructuredList) ConvertToNative(typeDesc reflect.Type) (interface{}, error) {
 	switch typeDesc.Kind() {
 	case reflect.Slice:
-		return t.elements, nil
+		switch t.itemsSchema.Type() {
+		// Workaround for https://github.com/kubernetes/kubernetes/issues/117590 until we
+		// resolve the desired behavior in cel-go via https://github.com/google/cel-go/issues/688
+		case "string":
+			var result []string
+			for _, e := range t.elements {
+				s, ok := e.(string)
+				if !ok {
+					return nil, fmt.Errorf("unexpected all elements to be of type string, but got %T", e)
+				}
+				result = append(result, s)
+			}
+			return result, nil
+		default:
+			return t.elements, nil
+		}
 	}
 	return nil, fmt.Errorf("type conversion error from '%s' to '%s'", t.Type(), typeDesc)
 }