This regression came about because the vtable deep copy for derived
types with unlimited polymorphic components was not making use of the
_len parameter to compute the memory to be allocated and the offsets
to array elements.

The ChangeLogs are reasonably self explanatory.

Bootstraps and regtests on FC27/x86_64 - OK for trunk and 7-branch?

Paul

2018-03-11  Paul Thomas  <pault@gcc.gnu.org>

    PR fortran/84546
    * trans-array.c (structure_alloc_comps): Make sure that the
    vptr is copied and that the unlimited polymorphic _len is used
    to compute the size to be allocated.
    * trans-expr.c (gfc_get_class_array_ref): If unlimited, use the
    unlimited polymorphic _len for the offset to the element.
    (gfc_copy_class_to_class): Set the new 'unlimited' argument.
    * trans.h : Add the boolean 'unlimited' to the prototype.

2018-03-11  Paul Thomas  <pault@gcc.gnu.org>

    PR fortran/84546
    * gfortran.dg/unlimited_polymorphic_29.f90 : New test.