[Bug fortran/77878] New: Valid OpenMP reduction of allocatable array produces a warning with -Wrealloc-lhs

[ibethune at epcc dot ed.ac.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77878

            Bug ID: 77878
           Summary: Valid OpenMP reduction of allocatable array produces a
                    warning with -Wrealloc-lhs
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ibethune at epcc dot ed.ac.uk
  Target Milestone: ---

An OpenMP fortran code which declares an allocatable array as an OpenMP
reduction variable generates a warning when compiled with -Wrealloc-lhs. I have
observed this with gfortran 6.1.0.

A simple example to show this:

mbp-ib:reduction ibethune$ cat reduction.f90 
program reduction
double precision, dimension(:), allocatable :: a
allocate(a(3))
!$OMP PARALLEL REDUCTION(+:a)
!$OMP END PARALLEL
deallocate(a)
end program
mbp-ib:reduction ibethune$ gfortran -Wrealloc-lhs -fopenmp -o reduction
reduction.f90
reduction.f90:4:29:

 !$OMP PARALLEL REDUCTION(+:a)
                             1
Warning: Code for reallocating the allocatable array at (1) will be added
[-Wrealloc-lhs]
mbp-ib:reduction ibethune$ gfortran -Wrealloc-lhs -o reduction reduction.f90
mbp-ib:reduction ibethune$

It's desirable to have this warning flag enabled to guard against poor code
style elswhere in the codebase e.g. array assignment without (:) notation. 
Would it be possible to either:

* Have the OpenMP directives translated into code which does not trigger this
warning

or

* Have the warning silenced when it comes from OpenMP-generated code rather
than user code.

Thanks!