TMyClass = class
public
  class var Test: integer;
  class constructor Create;
end;

{ TMyClass }

class constructor TMyClass.Create;
begin
  Test:=0;
end;

begin
  inc(TMyClass.Test);
end.

procedure TMyClass.Test;
{$WRITEABLECONST ON}
const
  CIdx: Integer = 5;
{$WRITEABLECONST OFF}
begin
  Inc(CIdx);
  ShowMessageFmt('%d', [CIdx]);
end;

procedure TMyClass.Test2;
{$WRITEABLECONST ON}
const
  CIdx: Integer = 6;
{$WRITEABLECONST OFF}
begin
  Inc(CIdx);
  ShowMessageFmt('%d', [CIdx]);
end;