TCustomAttribute and class internal types

TCustomAttribute and class internal types

Consider

Default = class(TCustomAttribute)
  DefaultVal: Integer;
  constructor Create(const aVal:Integer);
end;

TMyBaseClass = class(TObject)
public
   constructor Create; virtual;  // handles attributes
end;

// 1
TMyClass = class(TMyBaseClass)
public
  [Default(5)]
  Value: Integer;
end;

which is ok, versus

TMyBaseClass = class(TObject)
public
   type
      Default = class(TCustomAttribute)
        DefaultVal: Integer;
        constructor Create(const aVal:Integer);
      end;

   constructor Create; virtual;  // handles attributes
end;

// 2
TMyClass = class(TMyBaseClass)
public
  [Default(5)]  
W1025 Unsupported language feature: 'custom attribute'
  Value: Integer;
end;

as well as

// 3
TMyClass = class(TMyBaseClass)
public
  [TMyBaseClass.Default(5)]  
E2010 Incompatible types: 'TQuerySet' and 'TCustomAttribute'
  Value: Integer;
end;

Question
I can understand why //2 is a fail, since internal types have to be qualified, but why is //3 not a compatible type?

Update: Feature Request added to QP
https://quality.embarcadero.com/browse/RSP-13512