From 5f76b80fcc455a4fd8ece2713e8d043e76ded49b Mon Sep 17 00:00:00 2001
From: Sokwhan Huh <sokwhan@google.com>
Date: Tue, 19 Sep 2023 22:11:27 +0000
Subject: [PATCH 1/2] Fix last optional element to be marked as an optional
 index while folding

---
 cel/folding.go      | 11 +++++++----
 cel/folding_test.go | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/cel/folding.go b/cel/folding.go
index 025b2b04..07ac239e 100644
--- a/cel/folding.go
+++ b/cel/folding.go
@@ -248,23 +248,26 @@ func pruneOptionalListElements(ctx *OptimizerContext, e ast.Expr) {
 	}
 	updatedElems := []ast.Expr{}
 	updatedIndices := []int32{}
-	for i, e := range elems {
-		if !l.IsOptional(int32(i)) {
+	newOptIndex := 0
+	for _, e := range elems {
+		newOptIndex++
+		if !l.IsOptional(int32(newOptIndex)) {
 			updatedElems = append(updatedElems, e)
 			continue
 		}
 		if e.Kind() != ast.LiteralKind {
 			updatedElems = append(updatedElems, e)
-			updatedIndices = append(updatedIndices, int32(i))
+			updatedIndices = append(updatedIndices, int32(newOptIndex))
 			continue
 		}
 		optElemVal, ok := e.AsLiteral().(*types.Optional)
 		if !ok {
 			updatedElems = append(updatedElems, e)
-			updatedIndices = append(updatedIndices, int32(i))
+			updatedIndices = append(updatedIndices, int32(newOptIndex))
 			continue
 		}
 		if !optElemVal.HasValue() {
+			newOptIndex-- // Skipping causes the list to get smaller.
 			continue
 		}
 		e.SetKindCase(ctx.NewLiteral(optElemVal.GetValue()))
diff --git a/cel/folding_test.go b/cel/folding_test.go
index 18451ef6..d21d9244 100644
--- a/cel/folding_test.go
+++ b/cel/folding_test.go
@@ -129,10 +129,26 @@ func TestConstantFoldingOptimizer(t *testing.T) {
 			expr:   `[google.expr.proto3.test.TestAllTypes{single_int32: 2 + 3}].map(i, i)[0]`,
 			folded: `google.expr.proto3.test.TestAllTypes{single_int32: 5}`,
 		},
+		{
+			expr:   `[?optional.ofNonZeroValue(0)]`,
+			folded: `[]`,
+		},
 		{
 			expr:   `[1, ?optional.ofNonZeroValue(0)]`,
 			folded: `[1]`,
 		},
+		{
+			expr:   `[optional.none(), ?x]`,
+			folded: `[optional.none(), ?x]`,
+		},
+		{
+			expr:   `[?optional.none(), ?x]`,
+			folded: `[?x]`,
+		},
+		{
+			expr:   `[1, x, ?optional.ofNonZeroValue(0), ?x.?y]`,
+			folded: `[1, x, ?x.?y]`,
+		},
 		{
 			expr:   `[1, x, ?optional.ofNonZeroValue(3), ?x.?y]`,
 			folded: `[1, x, 3, ?x.?y]`,

From f3950e995c467488b929d8d5812da2280556fc36 Mon Sep 17 00:00:00 2001
From: Sokwhan Huh <sokwhan@google.com>
Date: Tue, 19 Sep 2023 22:18:34 +0000
Subject: [PATCH 2/2] Start the index at -1

---
 cel/folding.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cel/folding.go b/cel/folding.go
index 07ac239e..1ab0023a 100644
--- a/cel/folding.go
+++ b/cel/folding.go
@@ -248,7 +248,7 @@ func pruneOptionalListElements(ctx *OptimizerContext, e ast.Expr) {
 	}
 	updatedElems := []ast.Expr{}
 	updatedIndices := []int32{}
-	newOptIndex := 0
+	newOptIndex := -1
 	for _, e := range elems {
 		newOptIndex++
 		if !l.IsOptional(int32(newOptIndex)) {