Contraint error when "out" parameter has incorrect initial content

Contraint error when "out" parameter has incorrect initial content

[pascal.malaise]

Hi,
I am wondering if GNAT is correct when raising Constraint_Error in the following case:
procedure Out_Discr is

  type T (B : Boolean := False) is record
    case B is
      when True => null;
      when False => null;
    end case;
  end record;
  subtype Tt is T(True);

  procedure P (V : out Tt) is
  begin
    V := (B => True);
  end P;

  M : T;

begin
  M := (B => False);
  P (M); -- <-- Here
end Out_Discr;

>> raised CONSTRAINT_ERROR : out_discr.adb:20 discriminant check failed
Indeed M is not Tt when calling P, but M is "out" parameter. Shouldn't it be overwritten without any constraint check?

Thank you.

Re: Contraint error when "out" parameter has incorrect initial content

[AdaMagica]

RM 6.4.1(16) A formal parameter of mode in out or out with discriminants is constrained if either its nominal subtype or the actual parameter is constrained.

Thus the compiler may assume that variables supplied to calls already have the correct subtype.

Re: Contraint error when "out" parameter has incorrect initial content

[Niklas Holsti]

On 13-08-25 17:31 , AdaMagica wrote:
> RM 6.4.1(16) A formal parameter of mode in out or out with
> discriminants is constrained if either its nominal subtype or the
> actual parameter is constrained.
> 
> Thus the compiler may assume that variables supplied to calls already
> have the correct subtype.

In other words, as I understand it, this discriminant-check and
Constraint_Error happen when the parameter is passed into the call,
before the assignment statement in the callee.

The discriminant-check occurs because the formal parameter is
constrained to the subtype with a True discriminant. Since the actual
parameter has a False discriminant, the check fails and Constraint_Error
is raised.

Sloppily said: even if the formal parameter mode is "out", if the formal
constrains the discriminants then the discriminants are "in".

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .

Re: Contraint error when "out" parameter has incorrect initial content

[AdaMagica]

On Sunday, August 25, 2013 4:49:41 PM UTC+2, Niklas Holsti wrote:
> Sloppily said: even if the formal parameter mode is "out", if the formal
> constrains the discriminants then the discriminants are "in".

The discriminants have always been in (as are ranges of arrays). In Ada 83, out parameters were not allowed to be read, but discriminants and bounds of arrays were readable.

Re: Contraint error when "out" parameter has incorrect initial content

[pascal.malaise]

Thank you very much for the reference to the RM and of the explanation.