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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Is this a bug in my code or the compiler?
Date: Sat, 13 Jun 2015 22:10:34 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:26313
Date: 2015-06-13T22:10:34-05:00
List-Id: <comp.lang.ada>

"David Botton" <david@botton.com> wrote in message 
news:4f4cd4b1-0a6d-441b-a4f7-98add70e4e1e@googlegroups.com...
> Given:
>
>   function Token_Start (Source : in out Awesome.Source.Source_Type'Class)
>                         return Character;
>   function Token_End (Source : in out Awesome.Source.Source_Type'Class)
>                       return String;
>
> The following works:
>
>   function Get_Token_Text (Source : in out 
> Awesome.Source.Source_Type'Class)
>                            return String
>   is
>      N : Character := Token_Start (Source);
>   begin
>      return N & Token_End (Source);
>   end Get_Token_Text;
>
> The following does not work:
>
>   function Get_Token_Text (Source : in out 
> Awesome.Source.Source_Type'Class)
>                            return String
>   is
>   begin
>      return Token_Start (Source) & Token_End (Source);
>   end Get_Token_Text;
>
> Token_End is never called and only the value of Token_Start.

I would have expected this second expression to be illegal, because it seems 
to violate 6.4.1(6.18-19/3). That rule was introduced in Ada 2012 along with 
"in out" function parameters to avoid "obvious" dependence on order of 
evaluation.

The rule is:

If a construct C has two or more direct constituents that are names or 
expressions whose evaluation may occur in an arbitrary order, at least one 
of which contains a function call with an in out or out parameter, then the 
construct is legal only if:

For each name N that is passed as a parameter of mode in out or out to some 
inner function call C2 (not including the construct C itself), there is no 
other name anywhere within a direct constituent of the construct C other 
than the one containing C2, that is known to refer to the same object.

(Here C is "&", N is Source, and C2 is Token_Start; the rule says that 
Source can't appear outside of the call of Token_Start, and it does. I'd 
suggest filing a bug report.)

                            Randy.