private const SKIP_VAR_CHECK_TYPES = [
        ast\AST_ARG_LIST       => true,  // may be a reference
        ast\AST_ARRAY_ELEM     => true,  // [$x, $y] = expr() is an AST_ARRAY_ELEM. visitArray() checks the right-hand side.
        ast\AST_ASSIGN_OP      => true,  // checked in visitAssignOp
        ast\AST_ASSIGN_REF     => true,  // Creates by reference?