This fix is straightforward and described by the ChangeLog. Jose Rui
Faustino de Sousa posted the same fix for the ICE on the fortran list
slightly more than three years ago. Thinking that he had commit rights, I
deferred but, regrettably, the patch was never applied. The attached patch
also fixes storage_size and transfer for unlimited polymorphic arguments
with character payloads.

OK for mainline and backporting after a reasonable interval?

Paul

Fortran: Unlimited polymorphic intrinsic function arguments [PR84006]

2024-05-08  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
PR fortran/84006
PR fortran/100027
PR fortran/98534
* trans-expr.cc (gfc_resize_class_size_with_len): Use the fold
even if a block is not available in which to fix the result.
(trans_class_assignment): Enable correct assignment of
character expressions to unlimited polymorphic variables using
lhs _len field and rse string_length.
* trans-intrinsic.cc (gfc_conv_intrinsic_storage_size): Extract
the class expression so that the unlimited polymorphic class
expression can be used in gfc_resize_class_size_with_len to
obtain the storage size for character payloads. Guard the use
of GFC_DECL_SAVED_DESCRIPTOR by testing for DECL_LANG_SPECIFIC
to prevent the ICE. Also, invert the order to use the class
expression extracted from the argument.
(gfc_conv_intrinsic_transfer): In same way as 'storage_size',
use the _len field to obtaining the correct length for arg 1.

gcc/testsuite/
PR fortran/84006
PR fortran/100027
* gfortran.dg/storage_size_7.f90: New test.

PR fortran/98534
* gfortran.dg/transfer_class_4.f90: New test.