!standard 4.9(20)                                  20-06-16  AI12-0385-1/02
!class Amendment 20-06-10
!status Amendment 1-2012 20-06-16
!status ARG Approved 13-0-0  20-06-13
!status work item 20-06-10
!status received 20-06-09
!priority Low
!difficulty Easy
!subject Predefined shifts and rotates should be static 
!summary

The predefined shifts and rotates are considered static functions.

!problem

The shifting and rotating functions declared in package Interfaces are not
static. That prevents using them in case alternatives, named numbers, bounds
of integer type declarations, and so on. These can be useful to match a 
foreign type or subprogram definition.

!proposal

(See Summary.)

!wording

Add after RM 4.9(20):

  * a shifting or rotating function associated with a modular
     type declared in package Interfaces (see B.2);

!discussion

This change is incompatible on the margins. Static expressions are illegal
if they raise any predefined exceptions; that's not true of other expressions.
If an expression that raises an exception is changed to be static, the program
will be rejected. If the code in question will not be executed, that can be
a problem.

For example:

    declare
       Shift : constant := 4;
       Base : constant Interfaces.Unsigned_32 := 7;
    begin
       if Shift < 4 then
           return 256 / Interfaces.Shift_Right (Base, Shift);
       else
           return 1;
       end if;
    end;

If the Shift_Right call is a evaluated as a static function, this code is
illegal as we have a static divide-by-zero.


We could have added an Implementation Permission in 6.8 similar to:

  An implementation may allow the aspect Static on a function for which the
  convention is Intrinsic and the formal parameters and result types are 
  static scalar or string subtypes, signifying that the function is a 
  static function (see 4.9).

However, given that a static expression can be illegal while the same 
expression that is non-static is legal, such a permission would involve some 
loss of portability (especially important in code that will not be executed,
as noted above) and incompatibility with previous versions of the language.

!corrigendum 4.9(20)

@dinsa
@xbullet<a predefined concatenation operator whose result type is a string
type;>
@dinst
@xbullet<a shifting or rotating function associated with a modular
type declared in package Interfaces (see B.2);>

!ASIS

No ASIS effect.

!ACATS test

An ACATS C-Test is needed to check that these functions can be used in
static expressions.

!appendix

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