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Consider type Use = record public class function When<T>(const aBool: Boolean; const WhenTrue: T; const WhenFalse: T): T; static; inline; end; { Use } class function Use.When<T>(const aBool: Boolean; const WhenTrue, WhenFalse: T): T; begin if aBool then Result := WhenTrue else Result := WhenFalse end; procedure Test(Cond: Boolean); type TObjectClass = class of TObject; var i: Integer; b: Byte; c: Cardinal; w: Word; s: String; d: Double; o: TObjectClass; begin s := Use.When(Cond,'True', 'False'); s := Use.When(Cond, 1, 2).ToString; i := Use.When<Integer>(Cond, 1, -2); b := Use.When<Byte>(Cond, 1, 2); c := Use.When(Cond, 1, 2); w := Use.When(Cond, 1, 2); d := Use.When(Cond, 3.14, 42.0); o := Use<TObjectClass>.When(Cond, TObject, TStringList); end; This is valid code. But i := Use.When<Integer>(Cond, 1, -2); requires the type to be specified. Also w := Use.When(Cond, 1, 128); stops working - actually for any signed or unsigned type - when the second parameter is changed to 128 or higher. It then complains: Why? I though the left hand type would assist in the type inference and hence the parameter validation? Edit: I just noticed that this only happens for Error Insight - not during compilation.