Added SYSTEM_USER handling

Changelog.md

@@ -11,6 +11,7 @@ Support new syntax of PostgreSQL 16 (as of beta 3)
      `nodes\json\JsonObjectAgg`;
    * Constructor functions `json_array()` and `json_object()` represented by
      `nodes\json\JsonArrayValueList`, `nodes\json\JsonArraySubselect`, `nodes\json\JsonObject` classes.
+ * `SYSTEM_USER` server variable backed by `nodes\expressions\SQLValueFunction`.
 
 
 ## [2.2.0] - 2023-05-14

src/sad_spirit/pg_builder/nodes/expressions/SQLValueFunction.php

@@ -51,6 +51,7 @@ class SQLValueFunction extends GenericNode implements FunctionLike, ScalarExpres
     public const USER              = 'user';
     public const CURRENT_CATALOG   = 'current_catalog';
     public const CURRENT_SCHEMA    = 'current_schema';
+    public const SYSTEM_USER       = 'system_user';
 
     public const CURRENT_TIME      = 'current_time';
     public const CURRENT_TIMESTAMP = 'current_timestamp';
@@ -67,7 +68,8 @@ class SQLValueFunction extends GenericNode implements FunctionLike, ScalarExpres
         self::SESSION_USER,
         self::USER,
         self::CURRENT_CATALOG,
-        self::CURRENT_SCHEMA
+        self::CURRENT_SCHEMA,
+        self::SYSTEM_USER
     ];
 
     /**

tests/ParseFunctionCallTest.php

@@ -105,7 +105,7 @@ protected function setUp(): void
     public function testNoParenthesesFunctions(): void
     {
         $list = $this->parser->parseExpressionList($input = <<<QRY
-    current_date, current_role, current_user, session_user, user, current_catalog, current_schema
+    current_date, current_role, current_user, session_user, user, current_catalog, current_schema, system_user
 QRY
         );
         $expected = [];