[Bug c++/57693] The program logically failed in case of used "int b += b++"

["manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57693

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
> Both:
>  b += b++;
> And
>         d += d++;
> 
> are undefined what value b and d is going to be as there are no sequence
> point intbetween the two assignments.
> 
> They could be:
> temp = b;
> b++;
> b += temp;
> 
> or b++;
> b+=b;

Please, could we make more use of the FAQ? 

http://gcc.gnu.org/wiki/FAQ#undefinedbut

If you think the answer does not provide enough information, just extend the
answer there. (it could mention some example, and -Wsequence-points).

Otherwise, we have the same back and forth over and over again to the
frustration of users and the time waste of developers: what is UB? but my
program worked before! but my program works without optimizations! but I know
how it should work! can't gcc be consistent? but UB should not affect my
problem! I think this should not be UB!
>From gcc-bugs-return-424969-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Mon Jun 24 10:15:22 2013
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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/57696] New: Defined assignment for components not used when those are ALLOCATABLE
Date: Mon, 24 Jun 2013 10:15:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW696

            Bug ID: 57696
           Summary: Defined assignment for components not used when those
                    are ALLOCATABLE
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: pault at gcc dot gnu.org

The following prints
          42
          42
with gfortran and
 42
 20
with crayftn. Thus, gfortran does not use defined assignment - but it should.

Note: If "right%foo" is not allocated, the code segfaults with crayftn.


module m0
  implicit none
  type component
    integer :: i = 42
  contains
    procedure :: assign0
    generic :: assignment(=) => assign0
  end type
  type parent
    type(component), allocatable :: foo
  end type
contains
  elemental subroutine assign0(lhs,rhs)
    class(component), intent(INout) :: lhs
    class(component), intent(in) :: rhs
    lhs%i = 20
  end subroutine
end module

program main
  use m0
  implicit none
  type(parent) :: left, right
  allocate(right%foo)
  print *, right%foo
  left = right
  print *, left%foo
end