[flang] Handle passing NULL() to polymorphic pointer argument

[clementval]

Summary

Only updates the assert to check where the box is a BaseBoxType
instead of a BoxType since ClassType are also valid in that case.

flang/lib/Lower/ConvertExpr.cpp

for (const Fortran::lower::CallInterface<
           // If expr is NULL(), the mutableBox created must be a deallocated
           // pointer with the dummy argument characteristics (see table 16.5
           // in Fortran 2018 standard).
           // No length parameters are set for the created box because any non
           // deferred type parameters of the dummy will be evaluated on the
           // callee side, and it is illegal to use NULL without a MOLD if any
           // dummy length parameters are assumed.
           mlir::Type boxTy = fir::dyn_cast_ptrEleTy(argTy);
-          assert(boxTy && boxTy.isa<fir::BoxType>() &&
+          assert(boxTy && boxTy.isa<fir::BaseBoxType>() &&
                  "must be a fir.box type");
           mlir::Value boxStorage = builder.createTemporary(loc, boxTy);
           mlir::Value nullBox = fir::factory::createUnallocatedBox(
               builder, loc, boxTy, /*nonDeferredParams=*/{});
           builder.create<fir::StoreOp>(loc, nullBox, boxStorage);
           caller.placeInput(arg, boxStorage);
           continue;
         }

flang/test/Lower/polymorphic.f90

 ! CHECK:   %[[REBOX:.*]] = fir.rebox %[[P]] : (!fir.box<!fir.array<?xf32>>) -> !fir.class<!fir.array<?xnone>>
 ! CHECK:   fir.result %[[REBOX]] : !fir.class<!fir.array<?xnone>>
 ! CHECK: } else {
 ! CHECK:   %[[ABSENT:.*]] = fir.absent !fir.class<!fir.array<?xnone>>
 ! CHECK:   fir.result %[[ABSENT]] : !fir.class<!fir.array<?xnone>>
 ! CHECK: }
 ! CHECK: fir.call @_QMpolymorphic_testPsub_with_poly_array_optional(%[[BOX]]) {{.*}} : (!fir.class<!fir.array<?xnone>>) -> ()
 
+  subroutine pass_poly_pointer_optional(p)
+    class(p1), pointer, optional :: p
+  end subroutine
+
+  subroutine test_poly_pointer_null()
+    call pass_poly_pointer_optional(null())
+  end subroutine
+
+! CHECK-LABEL: func.func @_QMpolymorphic_testPtest_poly_pointer_null() {
+! CHECK: %[[ALLOCA:.*]] = fir.alloca !fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>>
+! CHECK: %[[ZERO:.*]] = fir.zero_bits !fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>
+! CHECK: %[[EMBOX:.*]] = fir.embox %[[ZERO]] : (!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>) -> !fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>>
+! CHECK: fir.store %[[EMBOX]] to %[[ALLOCA]] : !fir.ref<!fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>>>
+! CHECK: fir.call @_QMpolymorphic_testPpass_poly_pointer_optional(%[[ALLOCA]]) fastmath<contract> : (!fir.ref<!fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>>>) -> ()
+
 end module
 
 program test
   use polymorphic_test
   type(outer), allocatable :: o
   integer :: i(5)
   logical :: l(5)
   allocate(o)