Discriminant computation problem

[Sandro Magi <smagi@naasking.homeip.net>]

Just learning Ada and I came across this little issue. I can't perform a
computation on a discriminant. Suppose I want to create a bit vector type
that uses an array of Integers as its underlying storage type (just an
example). I want the user to provide the length of the bit vector in bits,
but I need to define the upper bound on the Integer array in Integer'Size
units.

type Int_Array is array (Positive range <>) of Integer;

type Bit_Vector (Max_Bit_Size : Positive) is
record
 Series : Int_Array (Positive'First .. Max_Bit_Size/Integer'Size);
end record;

The compiler error gnat produces: "discriminant in constraint must appear
alone". Is there something I'm doing wrong, or is the above simply not
possible?

I can solve this either by breaking the encapsulation of my type so that the
user must perform the computation ahead of time (not preferable), or by
creating a special function which computes the real upper bound. So the
user would have to instantiate:

bv : Bit_Vector(Real_Upper_Bound(1024)); --1024 bits

Instead of:

bv : Bit_Vector(1024); --1024 bits

Does anyone have a better answer?