Re: Incomplete type generic formal interactions with Implicit_Dereference

["Randy Brukardt" <randy@rrsoftware.com>]

Yes, I believe your example is legal. (I didn't actually try compiling it to 
see if there is any other problem.)

                  Randy.

"Jere" <jhb.chat@gmail.com> wrote in message 
news:97b909da-a35b-45f9-aa88-ad7cc192a1c9@googlegroups.com...
> On Monday, January 22, 2018 at 8:08:59 PM UTC-5, Randy Brukardt wrote:
>> "Jere" wrote in message
>> ...
>> > Part of me thinks that since Some_Type is incomplete, that Ada wouldn't
>> > allow for an Implicit_Dereference declared on it.  I guess it depends 
>> > on
>> > if it is meant to be evaluated in the compilation of the generic itself
>> > or at the point at which the function is called.
>> >
>> > Anyone have any thoughts on this?
>>
>> I don't think we ever considered this case. But I think it is OK.
>>
>> Specifically, 4.1.5(6/3) says that a generalized reference denotes a view
>> equivalent to that of a dereference of the reference discriminant of the
>> reference object. Thus, any Legality Rules that apply to the dereference 
>> of
>> the discriminant apply to the generalized reference as well. For 
>> instance,
>> this includes any accessibility rules (which was an important part of the
>> design, as the accessibility rules prevent the access value from living
>> longer than the object containing the access discriminant).
>>
>> As such, any other Legality Rules, like the ones about incomplete types
>> would also apply. Specifically, 3.10.1(10/3): "A prefix that denotes an
>> object shall not be of an incomplete view."
>>
>> So a use of an Implicit_Dereference wherever the type is incomplete would
>> usually be illegal (there are a few ways that a dereference of an 
>> incomplete
>> view can be used, mainly to be passed as a parameter to some routine with 
>> an
>> access-to-incomplete parameter). But it would be OK where the type has 
>> been
>> completed (in this case, outside of the generic instantiation, the way 
>> you
>> intended to use this). This sounds like what you intended.
>>
>>                                    Randy.
>
> I think I gotcha.  Just wanted to verify since I realized that I presented
> the example a bit disjointed.  If I have a dummy example:
>
> procedure Main is
>   generic
>      type Some_Item(<>);
>   package Some_Package is
>
>      type New_Type is tagged limited private;
>
>      type Some_Item_Holder(Ref : not null access Some_Item)
>      is limited null record
>         with
>            Implicit_Dereference => Ref;
>
>      function Get_Some_Item
>         (Object : New_Type)
>       return Some_Item_Holder;
>
>   private
>
>      type Some_Item_Access is access Some_Item;
>
>      type New_Type is tagged limited record
>         Some_Access : Some_Item_Access;
>      end record;
>
>   end Some_Package;
>
>
>
>   package body Some_Package is
>      function Get_Some_Item
>         (Object : New_Type)
>       return Some_Item_Holder
>      is
>      begin
>         return (Ref => Object.Some_Access);
>      end Get_Some_Item;
>   end Some_Package;
>
>   type Test;
>   package Test_Pkg is new Some_Package(Test);
>
>   type Test is record
>      value : Integer;
>   end record;
>
>   Thing : Test_Pkg.New_Type;
>
> begin
>   Thing.Get_Some_Item.Value := 4;
> end Main;
>
>
> From your last response, if I understand correctly, then this
> should be OK, legal Ada.  Sorry for the contrived example, I
> just wanted to make sure I didn't mislead with my original message,
> so I generated a minimal example.  I would normally have stuff
> broken up into separate files with better naming, etc.
>
> If Simon is seeing this, then the above code is one example of
> what causes one of the crashes.  It is the one that breaks in
> both versions (GPL 2017 and FSF 7.2):
>
>
> $ gcc --version
> gcc.exe (Rev1, Built by MSYS2 project) 7.2.0
> Copyright (C) 2017 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR 
> PURPOSE.
>
>
> $ gprbuild default.gpr
> gcc -c -gnatn -gnato -gnatE -fstack-check -gnat12 -O3 main.adb
> +===========================GNAT BUG 
> DETECTED==============================+
> | 7.2.0 (x86_64-w64-mingw32) Segmentation fault 
> |
> | Error detected at main.adb:49:9 
> |
> | Please submit a bug report; see https://gcc.gnu.org/bugs/ . 
> |
> | Use a subject line meaningful to you and us to track the bug. 
> |
> | Include the entire contents of this bug box in the report. 
> |
> | Include the exact command that you entered. 
> |
> | Also include sources listed below. 
> |
> +==========================================================================+
>
> Please include these source files with error report
> Note that list may not be accurate in some cases,
> so please double check that the problem can still
> be reproduced with the set of files listed.
> Consider also -gnatd.n switch (see debug.adb).
>
> Ada\Scratch\src\main.adb
>
> compilation abandoned
> gprbuild: *** compilation phase failed
>
>
> My NOTE:  the compilation flags just happened to be
> whatever they were last.  This is from my scratch project
> space.  I don't think they actually matter.
>