question reg usage of functional index as a on_conflict target

[AlekseiKhatkevich]

Hi
I have a functional index, like for example:

models.UniqueConstraint(fields=('field1', 'field2', Coalesce(F('field_3'), -1),
                                    name='uniq_index_name),

How can i use it as a targed for on_conflict clause?

AS 4 now it throws an exception
django.core.exceptions.SuspiciousOperation: Coalesce(F(field_3), Value(-1)) is not a valid conflict target, specify a list of column names, or tuples with column names and hstore key.