null exclusion and generics

[Georg Bauhaus <bauhaus@futureapps.de>]

If a generic formal type is private, I can supply a
null excluding subtype for the type parameter.
A generic formal private type means assignment is possible.
Declarations of objects of the null excluding
actual inside the generic need explicit initialization,
though.

generic
   type T is private;
procedure Nop(source: in T);

procedure Nop(source: in T) is
   tmp: T;  -- no known way to initialize
begin
   tmp := source;
end Nop;

Can I excpect, then, that Ada.Containers.* will typically
not be usable with null excluding subtypes? (Because an
implementation will likely declare local variables without
knowing how to initialize them.)

Is it a possible enhancement to add something like

generic
   type T is private;
   Default_Value: ...;  -- or a subprogram, or ...
...



-- Georg