Replace absl::nullopt with std::nullopt in p4-constraints

p4_constraints/ast.cc

@@ -113,7 +113,7 @@ absl::optional<int> TypeBitwidth(const Type& type) {
     case Type::kOptionalMatch:
       return type.optional_match().bitwidth();
     default:
-      return absl::nullopt;
+      return std::nullopt;
   }
 }
 

p4_constraints/backend/constraint_info.cc

@@ -120,7 +120,7 @@ absl::StatusOr<std::optional<ConstraintSource>> ExtractConstraint(
     }
   }
 
-  if (constraint_string.empty()) return absl::nullopt;
+  if (constraint_string.empty()) return std::nullopt;
 
   constraint_location.set_line(line);
   if (constraint_location.file_path().empty()) {
@@ -258,7 +258,7 @@ absl::StatusOr<TableInfo> ParseTableInfo(const Table& table) {
       std::optional<ConstraintSource> constraint_source,
       ExtractConstraint(ConstraintKind::kTableConstraint, table.preamble()));
 
-  std::optional<ast::Expression> constraint = absl::nullopt;
+  std::optional<ast::Expression> constraint = std::nullopt;
   if (constraint_source.has_value()) {
     ASSIGN_OR_RETURN(
         constraint,
@@ -341,7 +341,7 @@ std::optional<AttributeInfo> GetAttributeInfo(
   }
 
   // Unknown attribute.
-  return absl::nullopt;
+  return std::nullopt;
 }
 
 const TableInfo* GetTableInfoOrNull(const ConstraintInfo& constraint_info,

p4_constraints/backend/type_checker.cc

@@ -118,7 +118,7 @@ std::optional<Type> LeastUpperBound(const Type& left, const Type& right) {
   // While it is not true for partial orders in general that
   // LeastUpperBound(x,y) exists iff x >= y or y >= x, it is true for our
   // castability relation.
-  return absl::nullopt;
+  return std::nullopt;
 }
 
 // Mutates the input expression, wrapping it with a type_cast to the given type.