The best approach seems to be resolving the type inside create_block

src/allocator.f90

@@ -97,15 +97,23 @@ subroutine compute_padded_dims(self, sz)
 
   end subroutine
 
-  function create_block(self) result(ptr)
+  function create_block(self, next) result(ptr)
     !! Allocate memory for a new block and return a pointer to a new
     !! [[m_allocator(module):field_t(type)]] object.
     class(allocator_t), intent(inout) :: self
+    class(field_t), pointer, intent(in) :: next
     type(field_t), pointer :: newblock
     class(field_t), pointer :: ptr
     self%next_id = self%next_id + 1
     allocate (newblock)
-    newblock = field_t(self%ngrid, self%first, id=self%next_id)
+    associate(p_next => next)
+      select type (p_next)
+      type is (field_t)
+        newblock = field_t(self%ngrid, p_next, id=self%next_id)
+      class default
+        error stop "Incorrect overloading for create_block"
+      end select
+    end associate
     ptr => newblock
   end function create_block
 
@@ -130,7 +138,7 @@ function get_block(self, dir, data_loc) result(handle)
     if (.not. associated(self%first)) then
       ! Construct a field_t. This effectively allocates
       ! storage space.
-      self%first => self%create_block()
+      self%first => self%create_block(self%first)
     end if
     handle => self%first
     self%first => self%first%next ! 2nd block becomes head block

src/cuda/allocator.f90

@@ -79,14 +79,14 @@ function cuda_allocator_init(mesh, sz) result(allocator)
     allocator%allocator_t = allocator_t(mesh, sz)
   end function cuda_allocator_init
 
-  function create_cuda_block(self) result(ptr)
+  function create_cuda_block(self, next) result(ptr)
     class(cuda_allocator_t), intent(inout) :: self
-    type(cuda_field_t), pointer, intent(in) :: next
+    type(cuda_field_t), pointer :: next
     type(cuda_field_t), pointer :: newblock
     class(field_t), pointer :: ptr
     allocate (newblock)
     self%next_id = self%next_id + 1
-    newblock = cuda_field_t(self%ngrid, self%first, id=self%next_id)
+    newblock = cuda_field_t(self%ngrid, next, id=self%next_id)
     ptr => newblock
   end function create_cuda_block