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From: "Jeffrey R. Carter" <jrcarter@acm.org>
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Subject: Re: Brain bug or GNAT bug?
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Date: 2007-03-01T05:07:35+00:00
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(see below) wrote:
> 
>>>    generic 
>>>       type modular is mod <>;
>>>    package try is
>>>    
>>>       type DT1 (the_size : modular)  is limited private;
>>>    
>>>    -- subtype bounded is modular;
>>>       subtype bounded is modular range 1..9;

> But if the declaration of bounded compiles,
> the range of bounded must be a subset of the range of modular,
> so DT2.the_size must be in the range of modular.
> The rule at ARM 3.7(15) seems overly restrictive.

Consider:

with Ada.Command_Line;

procedure Modular_Subtype is
    type Modular is mod 2 ** 32;

    Low  : constant Modular :=
       Modular'Value (Ada.Command_Line.Argument (1) );
    High : constant Modular :=
       Modular'Value (Ada.Command_Line.Argument (2) );

    subtype Mod_Sub is Modular range Low .. High;

    generic -- Modular_Parameter
       type Modular is mod <>;
    package Modular_Parameter is
       subtype Bounded is Modular range 1 .. 9; -- Line 16
    end Modular_Parameter;

    package Instantiation is new Modular_Parameter (Modular => Mod_Sub);
begin -- Modular_Subtype
    null;
end Modular_Subtype;

This compiles (with a warning that Instantiation is not referenced). 
Since the instantiation cannot be checked during compilation, it is 
checked at run time. Here's a sample run:

D:\Code>modular_subtype 10 24

raised CONSTRAINT_ERROR : modular_subtype.adb:16 range check failed

Here's another:

D:\Code>modular_subtype 0 10

D:\Code>

-- 
Jeff Carter
"I unclog my nose towards you."
Monty Python & the Holy Grail
11