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From: Jeffrey Carter <jeffrey.carter@boeing.com>
Subject: Re: constrained subtypes
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Date: Tue, 12 Mar 2002 16:42:03 GMT
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Date: 2002-03-12T16:42:03+00:00
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George Stevens wrote:
> 
> Problem seemed to be in using:
> 
> case (constrained_subtype)
> 
> rather than:
> 
> case constrained_subtype
> 
> Removing the brackets sorted the problem.  Not sure why this is a problem
> for constrained subtypes, as the compilers cope fine for enums and (base)
> integers.

With parentheses, you have an expression, which is of the unconstrained
base subtype (Integer'Base), not the constrained subtype. Expressions
are always evaluated using the unconstrained base type. This allows
things such as

subtype X is Integer range 1 .. 8;
Y : Integer := 16;
Z : X;
...
Z := Y / 2;
Y := 3 * Z;

to be legal. "Y / 2" is not necessarily in X, so a constraint check is
performed before assignment. "3 * Z" is not of subtype X, but is legal,
even though Z is of subtype X, because it is an expression.

This is why "case Z" and "case (Z)" are different. "Case Y" and "case
(Y)" cover the same range, so you don't see a difference (though it's
there).

-- 
Jeffrey Carter