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From: Martin <martin.dowie@btopenworld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada2012 Invariants and obaque types
Date: Tue, 21 Jun 2011 05:01:48 -0700 (PDT)
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Date: 2011-06-21T05:01:48-07:00
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On Jun 21, 12:31=A0pm, Robert A Duff <bobd...@shell01.TheWorld.com>
wrote:
> I suggest you erase the invariant, fix the errors, and then put the
> invariant back in. =A0There's nothing wrong with your invariant.

Ok, with no Invariant:

package P1 is
   type T1 is tagged private;
   function Is_Valid (This : T1) return Boolean;
private
   type Imp;
   type Imp_Ref is not null access Imp;
   type T1 is tagged
      record
         I : Imp_Ref;
      end record;
end P1;

I get:
gnatmake -d -PH:\Ada\test_invarients\test_invarients.gpr p1.ads
gcc -c -g -g -gnatE -fstack-check -gnato -gnatf -fcallgraph-info=3Dsu,da
-gnat12 -I- -gnatA H:\Ada\test_invarients\src\p1.ads
cannot generate code for file p1.ads (package spec)
gnatmake: "H:\Ada\test_invarients\src\p1.ads" compilation error

[2011-06-21 12:58:10] process exited with status 4 (elapsed time:
00.37s)

i.e. no error.

The most reduced version I can come up with is now:
package P1 is
   type T1 is tagged private
     with Invariant =3D> True;  -- NB: not even a 'real' function
private
   type Imp;
   type Imp_Ref is not null access Imp;
   type T1 is tagged
      record
         I : Imp_Ref;
      end record;
end P1;

which produces the same original error:

gnatmake -d -PH:\Ada\test_invarients\test_invarients.gpr p1.ads
gcc -c -g -g -gnatE -fstack-check -gnato -gnatf -fcallgraph-info=3Dsu,da
-gnat12 -I- -gnatA H:\Ada\test_invarients\src\p1.ads
p1.ads:5:04: type "Imp" is frozen at line 2 before its full
declaration
gnatmake: "H:\Ada\test_invarients\src\p1.ads" compilation error

[2011-06-21 13:01:10] process exited with status 4 (elapsed time:
00.53s)

-- Martin