Pack using overloaded operator - fortran

Question

I am trying to use the pack function on an array of custom types. I have set up a small module with the type and interface to overload .eq.. If I do a simple comparison the overloaded operator appears to work, however when used in the context of a pack function I get an error.
!The module
module m_types
type :: t_property
character(12) :: key
logical :: value
end type t_property
type(t_property), allocatable, dimension(:) :: properties
public :: operator(.eq.)
interface operator(.eq.)
procedure prop_eq
end interface operator(.eq.)
contains
pure function prop_eq(first, second) result(res)
type(t_property), intent(in) :: first, second
logical :: res
if (first%key .eq. second%key) then
res = .true.
else
res = .false.
end if
end function prop_eq
end module m_types
! The test program
program textadventure
use m_types
implicit none
type(t_property) :: temp
allocate(properties(0))
temp = t_property(key="lit", value=.true.)
properties = [properties, temp]
temp = t_property(key="visited", value=.false.)
properties = [properties, temp]
temp = t_property(key="lit", value=.false.)
properties = [properties, temp]
temp = t_property(key="cold", value=.false.)
properties = [properties, temp]
temp = t_property(key="cold", value=.false.)
properties = [properties, temp]
print *, properties
print *, (properties(4) .eq. temp) ! Succeeds
print *, size(pack(properties, properties .eq. temp)) ! Fails
deallocate(properties)
end program textadventure
GCC error message
Error: Operands of comparison operator ‘.eq.’ at (1) are TYPE(t_property)/TYPE(t_property)
The spec says the mask in PACK should be a logical scalar which I thought I had provided - can someone point out where I am wrong?

Answer 1

In the comparison for the mask, properties .eq. temp you have two objects of type(t_property) and you hope to use the function prop_eq to provide that defined operation.
However, the first operand properties is an array, and the first dummy argument of prop_eq is a scalar. As a result, there is no defined operator .eq. available. You should provide a function to process an array first argument. One way would be to make prop_eq elemental.
Returning an array result from .eq. is appropriate: the mask= argument to PACK should conformable with the array argument (and you don't want it to be scalar).
In the successful comparison properties(4) .eq. temp the first operand is scalar.

Answer 2

The corrected code for those interested, it now packs as expected.
module m_types
type :: t_property
character(12) :: key
logical :: value
end type t_property
type(t_property), allocatable, dimension(:) :: properties
public :: operator(.eq.)
interface operator(.eq.)
procedure prop_eq
end interface operator(.eq.)
contains
function prop_eq(first, second) result(res)
type(t_property), intent(in) :: second
type(t_property), intent(in), dimension(:) :: first
logical, dimension(:), allocatable :: res
integer :: i
allocate(res(0))
do i=1, size(first)
if (first(i)%key .eq. second%key) then
res = [res, .true.]
else
res = [res, .false.]
end if
end do
end function prop_eq
end module m_types
program textadventure
use m_types
implicit none
type(t_property) :: temp
allocate(properties(0))
temp = t_property(key="cold", value=.false.)
properties = [properties, temp]
temp = t_property(key="lit", value=.true.)
properties = [properties, temp]
temp = t_property(key="visited", value=.false.)
properties = [properties, temp]
temp = t_property(key="lit", value=.false.)
properties = [properties, temp]
temp = t_property(key="cold", value=.false.)
properties = [properties, temp]
temp = t_property(key="cold", value=.false.)
properties = [properties, temp]
print *, size(properties)
print *,
print *, pack(properties, mask = properties .eq. temp)
deallocate(properties)
end program textadventure