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From: NCOHEN@IBM.COM (Norman Cohen)
Newsgroups: comp.lang.ada
Subject: (none)
Message-ID: <030889.085958.ncohen@ibm.com>
Date: 8 Mar 89 13:59:58 GMT
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Subject: Implicit conversion?

Ref: INFO-ADA Digest Volume 89 Issue 72 (Wed, Mar 8, 1989) Item #4
     (by Steven Holtsberg)

In the example

          with SYSTEM; use SYSTEM;

          procedure example is
            x: tiny_integer;
          begin
            x:= MAX_INT - MAX_INT;
          end example;

the two occurrences of the named number MAX_INT (which are "convertible
universal operands" as defined in RM 4.6(15)) are each converted to type
Tiny_Integer.  The "-" operator thus denotes the subtraction operation
for type Tiny_Integer.

Steven Holtsberg may have read too much into the rule (from RM 4.6(15)),
"An implicit conversion of a convertible universal operand is applied
if and only if the innermost complete context (see 8.7) determines
a unique (numeric) target type for the implicit conversion...."  This
determination can be indirect:  The innermost complete context determines
that the righthand side of the assignment must be of type Tiny_Integer;
since the only visible "-" operator with a result of type Tiny_Integer is
the "-" with operands of that type, the two operands must each be of type
Tiny_Integer; thus the conversion takes place.  In contrast, the complete
context

          Integer_Is_Symmetric: Boolean := Min_Int = -Max_Int;

does not determine a unique target type for a conversion.  Furthermore,
there is a "legal interpretation of this context without ... conversion"
(RM 4.6(15) again), so no conversion takes place.  The unary "-" with
an operand and result of type universal_integer is applied to Max_Int,
and the "=" operator with operands of type universal_integer and a
result of type Boolean is then applied to Min_Int and the result of the
negation.

Norman Cohen