Re: Generic formals and Aspects

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

<olivermkellogg@gmail.com> wrote in message 
news:18aedafc-c66d-4bba-81ce-76dce495f59e@googlegroups.com...
> On Tuesday, July 19, 2016 at 6:05:12 PM UTC+2, Shark8 wrote:
>> Try the following:
>>
>>
>> ------ SPEC ------
>>     generic
>> type Discrete_Type is (<>);  -- CANDIDATE
>>
>>     package Big_Endian_Integer_Buffer is
>>
>> function Get return Discrete_Type;
>> procedure Set (Value : Discrete_Type);
>>
>>     Private
>> type Internal is new Discrete_Type
>>   with Static_Predicate => Internal'Size in 16 | 32 | 64;

You could use a subtype here, if you don't want a new type:

  subtype Internal is Discrete_Type
      with Dynamic_Predicate => Internal'Size in 16 | 32 | 64;

BUT:

'Size shouldn't be allowed in either of these predicates, because "Internal" 
is a value (the value of the "current instance" of the subtype), while Size 
is the attribute of a subtype or object. (See AI12-0068-1.) This is 
necessary so that the properties of the object can't be queried in a 
predicate; that wasn't the purpose of predicates and it would allow some 
truly bizarre uses. (See "Zoofable" in the question of that AI.) 
Specifically, 8.6(17.1/4) says:

   Within an aspect_specification for a type or subtype, the current
   instance represents a value of the type; it is not an object. The
   nominal subtype of this value is given by the subtype itself (the
   first subtype in the case of a type_declaration), prior to applying
   any predicate specified directly on the type or subtype. If the type or
   subtype is by-reference, the associated object with the value is the
   object associated (see 6.2) with the execution of the usage name.

   AARM Ramification: For the purposes of Legality Rules, the current
   instance acts as a value within an aspect_specification. It might
   really be an object (and has to be for a by-reference type), but
   that isn't discoverable by direct use of the name of the current 
instance.

Looks like an ACATS test is needed.

>> Size_In_Bytes : constant Positive := Internal'Size / 8;
...
...
> That said, after changing Static_Predicate to Dynamic_Predicate in the 
> spec, the test program builds okay.
> However, it runs without failure. (I would have expected a failure on the 
> Fail instantiation.)

Did you remember to enable assertions? GNAT has the Assertion_Policy as 
Ignore by default. (This is implementation-defined in the Standard, mainly 
because we didn't have enough votes to make GNAT change.) If you're 
depending on assertions (like a predicate), you always need to appropriately 
place a Assertion_Policy pragma (or the equivalent command-line option, 
whatever it is).

                                       Randy.