Remove constraint that block initializer is non-empty.

PiperOrigin-RevId: 922222081

Author: jnthntatum

eval/compiler/flat_expr_builder.cc

@@ -1047,11 +1047,7 @@ class FlatExprVisitor : public cel::AstVisitor {
       }
       const auto& list_expr = call_expr.args().front().list_expr();
       block.size = list_expr.elements().size();
-      if (block.size == 0) {
-        SetProgressStatusError(absl::InvalidArgumentError(
-            "malformed cel.@block: list of bound expressions is empty"));
-        return;
-      }
+
       block.bindings_set.reserve(block.size);
       for (const auto& list_expr_element : list_expr.elements()) {
         if (list_expr_element.optional()) {
@@ -2052,7 +2048,9 @@ FlatExprVisitor::CallHandlerResult FlatExprVisitor::HandleBlock(
   }
 
   // Otherwise, iterative plan.
-  AddStep(CreateClearSlotsStep(block.index, block.slot_count, expr.id()));
+  if (block.slot_count > 0) {
+    AddStep(CreateClearSlotsStep(block.index, block.slot_count, expr.id()));
+  }
 
   return CallHandlerResult::kIntercepted;
 }

eval/compiler/flat_expr_builder_test.cc

@@ -2820,6 +2820,7 @@ TEST(FlatExprBuilderTest, BlockNotListOfBoundExpressions) {
 
 TEST(FlatExprBuilderTest, BlockEmptyListOfBoundExpressions) {
   ParsedExpr parsed_expr;
+  // Allowed, but degenerate case.
   ASSERT_TRUE(google::protobuf::TextFormat::ParseFromString(
       R"pb(
         expr: {
@@ -2835,10 +2836,8 @@ TEST(FlatExprBuilderTest, BlockEmptyListOfBoundExpressions) {
   CelExpressionBuilderFlatImpl builder(NewTestingRuntimeEnv());
   EXPECT_THAT(
       builder.CreateExpression(&parsed_expr.expr(), &parsed_expr.source_info()),
-      StatusIs(
-          absl::StatusCode::kInvalidArgument,
-          HasSubstr(
-              "malformed cel.@block: list of bound expressions is empty")));
+      StatusIs(absl::StatusCode::kInvalidArgument,
+               HasSubstr("invalid @index greater than number of bindings:")));
 }
 
 TEST(FlatExprBuilderTest, BlockOptionalListOfBoundExpressions) {