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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: expression functions and raise expressions
Date: Mon, 5 Mar 2018 14:21:15 -0600
Organization: JSA Research & Innovation
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Xref: reader02.eternal-september.org comp.lang.ada:50825
Date: 2018-03-05T14:21:15-06:00
List-Id: <comp.lang.ada>

If "Reference_Type" is a limited type, then this would have been correct at 
one point: we forgot to allow raise expressions in limited contexts. (Recall 
that returning a limited expression from a function is "built-in-place" and 
only allows certain expressions, copying objects [for instance] is not 
allowed.) I noticed this when writing some ACATS tests in this area, and it 
was fixed with a recent Binding Interpretation (correction). But it won't 
formally be adopted until Ada 2020, so an Ada 2012 compiler is ""correct" if 
it allows or rejects such a use (as corrections can, but don't have to be, 
applied to the existing language).

For a non-limited type, this should be just fine. But as a raise expression 
matches any type (and thus is different than any previous Ada expression), 
it might be buggy in some odd case.

                              Randy.

"Jere" <jhb.chat@gmail.com> wrote in message 
news:60f20df9-4b33-4f29-829e-2fccb89a650a@googlegroups.com...
>I wanted to double check if I am reading the RM correctly.
> From 6.8 2/4, I see that a function expression can consist entirely
> of an (expression)
>
> Section 4.4 2 says that an expression can be made up of a single
> (relation)
>
> Section 4.4 3/4 further says a relation can be a raise expression.
>
> Given that, I expect that the following declaration/definition is correct:
>
>      function Reference
>         (Container : aliased in out Container_Type;
>          Cursor    :                Cursor_Type)
>          return Reference_Type
>      is (raise My_Exception
>          with "Reference not supported for Ordered_Sets")
>      with Inline;
>
> GNAT GPL 2017 accepts it, but FSF GNAT 7.2 for mingw64 on Win10
> gives some errors:
> test.ads:328:11: (Ada 2005) cannot copy object of a limited type (RM-2005 
> 6.5(5.5/2))
> test.ads:328:11: return by reference not permitted in Ada 2005
>
> NOTE:  Reference_Type is untagged limited discriminated null
> record with Implicit_Dereference defined.  Also note that a similar
> function that returns an aggregate initialization compiles fine
> on both versions.
>
> I am compiling with Ada2012 mode (-gnat12 option)
>
> I believe this to be a bug in GCC 7.2 for mingw64, but wanted to
> make sure I wasn't misreading the RM.