From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM
	autolearn=unavailable autolearn_force=no version=3.4.4
X-Received: by 2002:a37:aa04:: with SMTP id t4mr77736810qke.359.1564504553353;
        Tue, 30 Jul 2019 09:35:53 -0700 (PDT)
X-Received: by 2002:a05:6808:b02:: with SMTP id
 s2mr55765355oij.155.1564504553047;
 Tue, 30 Jul 2019 09:35:53 -0700 (PDT)
Path: 
 eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!news.dns-netz.com!news.freedyn.net!newsreader4.netcologne.de!news.netcologne.de!peer02.ams1!peer.ams1.xlned.com!news.xlned.com!peer02.am4!peer.am4.highwinds-media.com!peer03.iad!feed-me.highwinds-media.com!news.highwinds-media.com!b26no6983330qtq.0!news-out.google.com!a5ni567qtd.0!nntp.google.com!b26no6983324qtq.0!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
Newsgroups: comp.lang.ada
Date: Tue, 30 Jul 2019 09:35:52 -0700 (PDT)
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com; posting-host=146.5.2.29;
 posting-account=lJ3JNwoAAAAQfH3VV9vttJLkThaxtTfC
NNTP-Posting-Host: 146.5.2.29
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <28e4ac2e-311b-40c4-ad81-42f55129ccd1@googlegroups.com>
Subject: SPARK prooving an array of Positives.
From: Shark8 <onewingedshark@gmail.com>
Injection-Date: Tue, 30 Jul 2019 16:35:53 +0000
Content-Type: text/plain; charset="UTF-8"
X-Received-Bytes: 3055
X-Received-Body-CRC: 1933741248
Xref: reader01.eternal-september.org comp.lang.ada:56983
Date: 2019-07-30T09:35:52-07:00
List-Id: <comp.lang.ada>

I have a bit of a problem getting the SPARK provers to accept that a postcondition cannot fail. Given the following in a spec file:

    Type Axis_Count is range 0..999   with Size => 10;
    Type Axis_Dimensions is Array (Axis_Count range <>) of Positive
      with Default_Component_Value => 1;
    Subtype Primary_Data_Array is Axis_Dimensions(1..999);
    Subtype Random_Groups_Data is Axis_Dimensions(1..998);

    Function EF( Item : FITS.Axis_Dimensions ) return Interfaces.Unsigned_64;

and the following in the implementation:

    Function EF( Item : FITS.Axis_Dimensions ) return Interfaces.Unsigned_64 is
        Max : Constant := Positive'Last;
        Function First return Interfaces.Unsigned_64 is
          ( Interfaces.Unsigned_64( Item( Item'First ) ) )
          with Inline, Pre => Item'Length > 0, Post => First'Result <= Max;
        Function Last return Interfaces.Unsigned_64 is
            ( Interfaces.Unsigned_64( Item( Item'Last ) ) )
          with Inline, Pre => Item'Length > 0, Post => Last'Result  <= Max;
        use all type Interfaces.Unsigned_64;
    Begin
	case Item'Length is
	   when 0      => return 1;
	   when 1      => return First;
	   when 2      => return First * Last;
	   when others =>
	    Declare
		Middle : Constant Axis_Count := Item'Length/2 + Item'First;
		Subtype Head is Axis_Count range Item'First..Middle;
		Subtype Tail is Axis_Count range Axis_Count'Succ(Middle)..Item'Last;
	    Begin
		Return EF(Item(Head)) * EF(Item(Tail));
	    End;
	end case;
    End EF;

the SPARK prover is issuing warnings that the postconditions might fail.
"medium: postcondition might fail, cannot prove First'Result <= Max (e.g. when First'Result = 0)"
But this is impossible given that the element-type is Positive and the precondition states there is at least one element. Does anyone know why this is happening? (And how to fix it?)