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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Lower bounds of Strings
Date: Tue, 5 Jan 2021 14:40:19 +0100
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On 2021-01-05 13:32, Jeffrey R. Carter wrote:
> On 1/5/21 12:57 PM, Dmitry A. Kazakov wrote:
>>
>> This is a constraint, a meaningless and dangerous one:
>>
>>     procedure Foo (X : Mono_String);
>>     S : String := "abcdefgh";
>> begin
>>     Foo (S (2..S'Last)); -- Boom! Constraint_Error
> 
> Surely the slice would slide, as it does in
> 
> with Ada.Text_IO;
> 
> procedure Slider is
>     subtype S7 is String (1 .. 7);
> 
>     procedure Foo (X : in S7);
> 
>     procedure Foo (X : in S7) is
>        -- Empty
>     begin -- Foo
>        Ada.Text_IO.Put (Item => X (7) );
>        Ada.Text_IO.New_Line;
>     end Foo;
> 
>     S : constant String := "abcdefgh";
> begin -- Slider
>     Foo (X => S (2 .. 8) );
> end Slider;
> 
> ~/Code$ gnatmake -gnatan -gnato2 -O2 -fstack-check slider.adb
> x86_64-linux-gnu-gcc-9 -c -gnatan -gnato2 -O2 -fstack-check slider.adb
> x86_64-linux-gnu-gnatbind-9 -x slider.ali
> x86_64-linux-gnu-gnatlink-9 slider.ali -O2 -fstack-check
> ~/Code$ ./slider
> h

Yes, but here S7 is definite, it is a quite different case. Sliding 
indefinite subtypes without copies, with access types allowed?

And even with definite subtypes it is broken:

    with Ada.Text_IO;  use Ada.Text_IO;

    procedure Main is
       subtype S7 is String (1..7);
       S : constant String := "abcdefgh";
       V : S7 renames S (2..8);
    begin
       Put_Line ("Is it broken? " & Boolean'Image (S (7) = V (7)));
    end Main;

This will print: Is it broken? TRUE

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de