Re: [PATCH,fortran] Relax restrictions on KIND in SELECT CASE constructs

[Steve Kargl <sgk at troutmask dot apl dot washington dot edu>]

On Mon, Dec 27, 2004 at 09:27:23AM -0800, Steve Kargl wrote:
> On Mon, Dec 27, 2004 at 01:46:16PM +0000, Paul Brook wrote:
> > > 2004-12-17  Steven G. Kargl  <kargls@comcast.net>
> > >
> > >      * resolve.c (validate_case_label_expr): Fix handling of mismatched
> > > KINDs.
> > 
> > No.
> 
> Okay, I'll open a PR about this problem.  I have no idea 
> how to fix this at the moment.
> 

I just re-read Pauls original reply and I think his suggestion 
is incorrect with respect to 7.2.3 of the standard.  I deleted
his email so, I'm reproducing it here:

> program foo
>   integer i
>   select case (i)
>   case (0_4)
>   case (1_8)
>   end select
> end program
> 
> Gives:
> 
>       SELECT CASE __convert_i4_i8[[((i))]]
>       CASE (0 0)
>       CASE (1_8 1_8)
>       END SELECT
> 
> Which is obviously wrong.
> 
> We should probably convert all cases to the type of the select
> expression.  Out of range values should either error, or warn
> and remove that case.

I think this is wrong because we need to convert "i == 1_8" to the 
KIND of i + 1_8 per 7.2.3, which has the kind of 1_8.  The passage
from the standard is

  In the numeric relational operation
         x1 rel-op x2
  if the types or kind type parameters of x1 and x2 differ, their values
  are converted to the type and kind type parameter of the expression
  x1 + x2 before evaluation.

Since the conversion must be performed before evaluation, we may now
conclude that in principle there are no out-of-range values.  Note,
this does not mean that certain cases are not unreachable.  For example,

   integer*1 i        ! kind = 1, -128 <= i < 127
   select case (i)    ! need to convert i to kind = 4.
   case (1)           ! kind = 4, reachable
   case (200)         ! kind = 4, unreachable because of range of i
   end select 

The question becomes more complicated with a slight variation of 
Paul's example.

   integer*1 i        ! kind = 1, -128 <= i < 127
   select case (i)    !
   case (1_4)         ! kind = 4, reachable
   case (2_8)         ! kind = 8, reachable
   end select 

Do we convert i and 1_4 to kind = 8?  Do we convert i to kind = 4
in the comparison with 1_4 and convert it to kind = 8 in the 
comparison with 2_8?   It is probably safe to convert everything to 
the largest kind, because the smaller kind are proper subsets
of the larger kinds. 

-- 
Steve