Re: bug or feature

[Adam Beneschan <adambeneschan@gmail.com>]

On Monday, September 8, 2014 1:52:01 AM UTC-7, Stephen Leake wrote:
> 
> >   1   with Ada.Text_IO;
> >   2   with Datum_Pkg;
> >   3  
> >   4   procedure Main is
> >   5 
> >   6      subtype Datum is Datum_Pkg.Datum;
> >   7      subtype Datum_Vector is Datum_Pkg.Datum_Vectors.Vector;
> >   8     
> >   9      D_Vec : Datum_Vector;
> >  10     
> >  11   begin
> >  12   
> >  13      for Dat : Datum_Pkg.Datum of D_Vec loop
> >  14         Ada.Text_IO.Put_Line ("Datum=" & Dat.Image);
> >  15      end loop;
> >  16      
> >  17      for Dat : Datum of D_Vec loop
> >  18         Ada.Text_IO.Put_Line ("Datum=" & Dat.Image);
> >  19      end loop;   
> >  20      
> >  21   end Main;
> 
> > Line 13 compiles without message but in line 17 I get an error message:
> >    subtype indication does not match element type
> > But this line should be equivalent to line 13 because of the subtype definition
> > in line 6.
> 
> line 6 defines a _new_ subtype, _different_ from Datum_Pkg.Datum. They have
> the same base type, but the error message says "subtype", not "base
> type".
> 
> So you are out of luck.
> 
> In general, a subtype can add constraints, which is why this matters; if
> the loop parameter has different constraints than the container object,
> you'll get runtime errors.

But that doesn't explain why he's getting a *compile-time* error.

As far as I can tell from the RM, you'd get an exception at run-time only if the subtype_indication is more restrictive than the container element *and* if the code actually tries to assign the loop parameter to an element that does not satisfy the subtype constraints.  This one looks like a GNAT bug to me.

                                   -- Adam