As well as the PR this patch fixes problems in handling class objects

2020-12-18  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/83118
	PR fortran/96012
	* resolve.c (resolve_ordinary_assign): Generate a vtable if
	necessary for scalar non-polymorphic rhs's to unlimited lhs's.
	* trans-array.c (get_class_info_from_ss): New function.
	(gfc_trans_allocate_array_storage): Defer obtaining class
	element type until all sources of class exprs are tried. Use
	class API rather than TREE_OPERAND. Look for class expressions
	in ss->info by calling get_class_info_from_ss. After, obtain
	the element size for class descriptors. Where the element type
	is unknown, cast the data as character(len=size) to overcome
	unlimited polymorphic problems.
	(gfc_conv_ss_descriptor): Do not fix class variable refs.
	(build_class_array_ref, structure_alloc_comps): Replace code
	replicating the new function gfc_resize_class_size_with_len.
	(gfc_alloc_allocatable_for_assignment): Obtain element size
	for lhs in cases of deferred characters and class enitities.
	Move code for the element size of rhs to start of block. Clean
	up extraction of class parameters throughout this function.
	After the shape check test whether or not the lhs and rhs
	element sizes are the same. Use earlier evaluation of
	'cond_null'. Reallocation of lhs only to happen if size changes
	or element size changes.
	* trans-expr.c (gfc_resize_class_size_with_len): New function.
	(gfc_get_class_from_expr): If a constant expression is
	encountered, return NULL_TREE;
	(trans_scalar_class_assign): New function.
	(gfc_conv_procedure_call): Ensure the vtable is present for
	passing a non-class actual to an unlimited formal.
	(trans_class_vptr_len_assignment): For expressions of type
	BT_CLASS, extract the class expression if necessary. Use a
	statement block outside the loop body. Ensure that 'rhs' is
	of the correct type. Obtain rhs vptr in all circumstances.
	(gfc_trans_scalar_assign): Call trans_scalar_class_assign to
	make maximum use of the vptr copy in place of assignment.
	(trans_class_assignment): Actually do reallocation if needed.
	(gfc_trans_assignment_1): Simplify some of the logic with
	'realloc_flag'. Set 'vptr_copy' for all array assignments to
	unlimited polymorphic lhs.
	* trans.c (gfc_build_array_ref): Call gfc_resize_class_size_
	with_len to correct span for unlimited polymorphic decls.
	* trans.h : Add prototype for gfc_resize_class_size_with_len.

gcc/testsuite/
	PR fortran/83118
	PR fortran/96012
	* gfortran.dg/dependency_60.f90: New test.
	* gfortran.dg/class_allocate_25.f90: New test.
	* gfortran.dg/class_assign_4.f90: New test.
	* gfortran.dg/unlimited_polymorphic_32.f03: New test.