Q:

How can I use TExtendedHelper on literals? 

With System. SysUtils. TShortIntHelper (and others) I can write:
output := 5.ToString();

to format the number 5 as string.  As well, there is System. SysUtls. TExtendedHelper, but I'm unable to compile:
output := (5.0).ToString();

E2018: Record, object or class type required

Other versions which don't work:

5.0.ToString()
(1.0+5.1).toString()
(5+0.).toString() (says E2029: ')' expected but ']' found)

Versions which do actually work:

(1+5.1).toString()
(1.1+1+5.1).toString()
5.9e0.toString()

If the extended value is declared const, it doesn't work either:
function TestFormat(): String;
const
  q = 5.5;
begin
  Result := q.ToString();
end;

But with a definition of q : extended = 5.5; it works.  So, I'm wondering why the compiler behaves this way. 

A:

You found errors in the compiler.  Please report it in Quality Portal. 
A workaround is to use the helpers class functions:
myString := Extended. ToString(5.5);

class function ToString(const Value: Extended): string; overload; inline; static;
class function ToString(const Value: Extended; const AFormatSettings: TFormatSettings): string; overload; inline; static;
class function ToString(const Value: Extended; const Format: TFloatFormat; const Precision, Digits: Integer): string; overload; inline; static;
class function ToString(const Value: Extended; const Format: TFloatFormat; const Precision, Digits: Integer;
                           const AFormatSettings: TFormatSettings): string; overload; inline; static;

