Error: container cannot be indexed with "Cursor"

[Blady <p.p11@orange.fr>]

Hello,

Container type UX is declared as:
    type UX is tagged private with
      Constant_Indexing => Constant_Reference, Variable_Indexing => 
Reference, Default_Iterator => Iterate,
      Iterator_Element  => Character,
      Aggregate => (Empty => Empty, Add_Unnamed => Append, New_Indexed 
=> New_Vector, Assign_Indexed => Replace_Element);


I declare:

S3 : UX;
...
80.    for I in S3.Iterate loop
82.       if F then
83.          S3(I) := 'e';
84.       end if;
85.       C := S3 (I);
87.    end loop;

GNAT 15 issues the errors:
test.adb:83:10: error: container cannot be indexed with "Cursor"
test.adb:85:14: error: container cannot be indexed with "Cursor"

Prior GNAT versions didn't issued any errors.

Is there a change in Ada standard? or GNAT?
Why is cursor indexing not possible?

Thanks for your answers, Pascal.