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From: "cmacmackin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/66409] New: Reporting ambiguous interface when overloading assignment with polymorphic array
Date: Wed, 03 Jun 2015 20:06:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66409

            Bug ID: 66409
           Summary: Reporting ambiguous interface when overloading
                    assignment with polymorphic array
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cmacmackin at gmail dot com
  Target Milestone: ---

I have come across what I believe to be a bug (or possibly a misunderstanding
of the standard on one of our counts) in gfortran. I'm trying to overload the
assignment operator with two custom procedures:

    module container_mod

        interface assignment(=)
            module procedure assign_to_variable
            module procedure assign_to_variable_1d
        end interface

    contains

        subroutine assign_to_variable (retval,input)
            class(*), intent(inout)         ::  retval
            real, intent(in)    ::  input

            return
        end subroutine assign_to_variable

        subroutine assign_to_variable_1d (retval_1d,input)
            class(*), dimension(:), intent(inout)   ::  retval_1d
            real, intent(in)            ::  input

            return
        end subroutine assign_to_variable_1d

    end module container_mod

(In the real program where I do this, the input variable is a derived type.)
However, I am getting the compile-time error 

    gfortran -Wall -c "dbg.f90"
    dbg.f90:5.46:
            module procedure assign_to_variable_1d
                                                  1
    Error: Ambiguous interfaces 'assign_to_variable_1d' and
'assign_to_variable' in intrinsic assignment operator at (1)

I've found that ifort 12.1.7.367 and pgfortran 8.0-6 both compile this just
fine. I've also found that, if I change the interface from assignment to just a
normal generic interface, it compiles properly. Finally, if I change the
unlimited polymorphic variables to anything else (a built-in variable type, a
derived type, or a non-unlimited polymorphic variable) it compiled without
problems.

It's also worth noting that, if I try to compile this with only the scalar
interface, gfortran complains when I try to use that to assign to an array. So
obviously the interface is not ambiguous when it comes to actually performing
the assignment.

I hope all of that's clear. I'll be happy to answer any questions or
clarifications.