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Subject: representation clauses ok in generics when the formal type in tagged
 with known ancestor,
 but not ok when only know to be tagged ? ... said the compiler.
From: Mehdi Saada <00120260a@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:51193
Date: 2018-03-24T10:16:15-07:00
List-Id: <comp.lang.ada>

This is a translation from fr.c.l.a.

I wonder why in some cases where a generic formal type is a tagged type and has a known ancestor, one can alter the representation:

package AAA is
   type Type_A is tagged
      record
         INT: INTEGER;
      end record;
end AAA;
with AAA; use AAA;
generic
   type NT is new type_A with private;
package TENTATIVE is
   type B_Type is new Nt with null record with SIZE => 300;
   procedure P1 (B : B_Type) is NULL;
end TENTATIVE;
with AAA, TENTATIVE; use AAA;
procedure MAIN is
package TEST is new TENTATIVE(Type_A);
begin Null; end; 

It only gives me "main.adb:3:01: warning: in instantiation at tentative.ads:5
main.adb:3:01: warning: 172 bits of "B_Type" unused"

It compiles, while when the contract states that NT has only to be tagged, it failed.
To change "type NT is new TYPE_A with private" into "type NT is tagged private" gives "tentative.ads:5:48: representation item not allowed for generic type"

JPRosen couldn't help on this, and said you Randy could saw to it better ;-)