derived formal types and known discriminants

[sbelmont700@gmail.com]

Can any language lawyer explain the reasoning behind precluding known discriminants for general formal derived types?  For example, I can write this:

generic
  type T is new P with private; -- constrained (i.e. none)
  
and I can write this:

generic
  type T (<>) is new P with private;  -- unconstrained (i.e. any)
  
But I can't write

generic
  type T (foo : Integer) is new P with private;  -- (just one integer)
  
presumably because 12.5.1~11/3 says "The declaration of a formal derived type shall not have a known_discriminant_part", to wit GNAT complains "discriminants not allowed for this formal type".  It would seems as though if unknown discriminants are legal, known would be an easier subset, and my interest has been piqued to the reason why.

Thank you for any insight

-sb