Seg fault on sourced allocation of polymorphic variable

When compiled with the “-g” option the following example seg faults on the sourced allocation statement. Interestingly, the code runs correctly without the “-g” option, however valgrind still shows bad/questionable things going on in the pgi runtime library.
This is with the new 18.10 community edition on 64-bit Linux.
The example is extracted from a unit test from https://github.com/nncarlson/yajl-fort

type, abstract :: json_value
end type

type, extends(json_value) :: json_string
  character(:), allocatable :: value
end type

class(json_value), allocatable :: x

call foo('fubar', x)
select type (x)
type is (json_string)
  if (x%value /= 'fubar') stop 1
class default
  stop 2
end select

contains

  subroutine foo(string, x)
    character(*), intent(in) :: string
    class(json_value), allocatable, intent(out) :: x
    allocate(x, source=json_string(string))  ! <== SEG FAULTS HERE
  end subroutine

end