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Date: Sat, 23 Sep 2023 13:22:09 -0700 (PDT)
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Subject: Valid attribute and input operations
From: Maciej Sobczak <see.my.homepage@gmail.com>
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Hi there,

I am in the middle of a heated debate with Richard Riehle on LinkedIn, wher=
e we cannot get to terms with regard to the exact semantics of X'Valid in t=
he context of input operations performed by standard Get procedure.
In short, consider the following example:

with Ada.Text_IO;
with Ada.Integer_Text_IO;
procedure Is_Valid_Test is
   X : Integer range 0..200;
begin
   Ada.Text_IO.Put("Get an Integer: ");
   Ada.Integer_Text_IO.Get(X);
   if X'Valid then
      Ada.Text_IO.Put_Line("The Input is Valid ");
   else
      Ada.Text_IO.Put_Line("The Input is not Valid ");
   end if;
end Is_Valid_Test;

When the input is 500, what should be the behavior of this program?
There are two interpretations:

1. Get is an input operation and can create invalid representations (as sta=
ted in 13.9.2, p.7). Then, the X'Valid test that follows Get(X) can be used=
 to safely recognize whether the value is in the range or not. The program =
should print the second string (from the else branch), but should not raise=
 any exceptions for this input (500).

2. Get is not an input operation in the meaning referred to in 13.9.2/7, or=
 is an input, but only for type Integer (and it cannot create invalid integ=
er representations on typical computers anyway). The X variable is an actua=
l parameter that has a subtype that is different from the formal parameter =
and is subject to conversions when the Get subprogram exits normally (6.4.1=
/17,17a). This conversion should raise Constraint_Error for this input (500=
).

I have checked the above program on several on-line compilers, all of them =
behave according to interpretation 2 above.
Richard claims to get behavior 1 on his compiler.

What is your take on this? Any language lawyers?

Regards,
Maciej Sobczak