Certain types are treated specially by the compiler on an internal basis in that they must be correctly finalized to release any resources that they might currently own. Because the compiler cannot determine what type is actually stored in a record's variant section at runtime, it is not possible to guarantee that these special data types are correctly finalized.

program Produce;

  type
    Data = record
      case kind:Char of
      'A': (str : String);
    end;

begin
end.

String is one of those types which requires special treatment by the compiler to correctly release the resources. As such, it is illegal to have a String in a variant section.

program Solve;

  type
    Data = record
      str : String;
    end;

begin
end.

One solution to this error is to move all offending declarations out of the variant section. Another solution would be to use pointer types (^String, for example) and manage the memory by yourself.