This problem was caused by the compiler attempting to use 0 as an lvalue
and to assign 0 to it. Understandably, this upset the gimplifer quite a bit
:-) The fix is to use the ss_info string length for deferred length
character components, where the hidden string length component has been
used. The use of a constant as an lvalue is prevented by checking that the
expression string length is a variable.

Regtests on FC33/x86_64 - OK for all three branches?

Paul

Fortran: Fix for class defined operators [PR99125].

2021-02-23  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
PR fortran/99125
* trans-array.c (gfc_conv_expr_descriptor): For deferred length
length components use the ss_info string length instead of
gfc_get_expr_charlen. Make sure that the deferred string length
is a variable before assigning to it. Otherwise use the expr.
* trans-expr.c (gfc_conv_string_length): Make sure that the
deferred string length is a variable before assigning to it.

gcc/testsuite/
PR fortran/99125
* gfortran.dg/alloc_deferred_comp_1.f90: New test.