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From: "kargl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/51208] [OOP] ALLOCATE with SOURCE= or MOLD=: Diagnose if variable occurs twice
Date: Fri, 18 Nov 2011 16:10:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51208

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kargl at gcc dot gnu.org

--- Comment #1 from kargl at gcc dot gnu.org 2011-11-18 15:53:51 UTC ---
(In reply to comment #0)
> In ALLOCATE, one gets a diagnosis if one uses:
> 
>    integer, allocatable :: i
>    allocate(i, stat=i)
>    end
> 
> Namely:
>    allocate(i, stat=i)
>                  1
>    Error: Stat-variable at (1) shall not be ALLOCATEd within
>           the same ALLOCATE statement

The above error is easy to produce, because gfortran
only has to check if an alloc-object is used as a
stat-variable.

> 
> However, for SOURCE= and MOLD= it does not work:
> 
>   type t
>     integer :: i = 4
>   end type t
>   class(t), allocatable :: x, y
> 
>   allocate(t :: y)
>   allocate(x, mold=x)   ! Not diagnosed
>   allocate(x, source=x) ! Not diagnosed

Doing a check here requires a walk of the mold-expr
and the source-expr, which is of course much more
work (unless gfortran already has a helper function
to check if a variable is used within an expression).

Here's a simple example where walking source-expr
may find the use of the alloc-object:

module bar
  real, allocatable :: x(:)
  contains
  function f(a)
    real f
    real :: a(:)
    f = sum(a)
  end function
end module bar

program foo
  use bar
  allocate(x(2), source=f(x))
  print *, x
end program foo
!
!program foo
!  use bar
!  allocate(x(2), source=f([1., 2.]))
!  print *, x
!end program foo


and here's a more complicated example.

module bar
  real, allocatable :: x(:)
  contains
  function f(a)
    real f
    real :: a
    f = sum(a * x)  ! x used un-allocated?
  end function
end module bar

program foo
  use bar
  allocate(x(2), source=f(2.))
  print *, x
end program foo