Why does FPC compile this? (Btw. regardless of mode.) program Project1; uses Classes; var O1, O2: TObject; B: Boolean; begin B := O1 is TPersistent or O2 is TPersistent; end. Delphi throws a compile error and according to FPC docs https://www.freepascal.org/docs-html/ref/refch12.html it should not compile in FPC either. Ondrej