Cannot use "ON CONFLICT" with postgres updatable view and partial index

I have an updatable view pointing to an underlying table that has a partial index. It looks something like this

CREATE TABLE if not exists foo (
    a INT,
    b INT,
    x INT,
    y INT,
    z BOOLEAN,
    CONSTRAINT x_or_y CHECK (
      (z and x is not null and y is null)
      or 
      (not z and x is null and y is not null)
    )
);
CREATE UNIQUE INDEX ux ON foo (x, a) WHERE z=TRUE;
CREATE UNIQUE INDEX uy ON foo (y, a) WHERE z=FALSE;
CREATE OR REPLACE VIEW  foo_view AS 
    SELECT * FROM foo;

That is, for each row, y must be null if z is true; x must be null if z is false; and, only one of x and y may be not null. (x, a) and (y, a) must be unique. (Sorry if this is overly complicated. I’m translating from my real table that has a lot of other cruft.)

My problem comes when I want to update with ON CONFLICT. I believe I ought to be able to do this.

INSERT INTO foo_view(x, y, a, b, z)
  VALUES
  (5, null, 1, 2, true),
  (null, 5, 1, 2, false);
  

  
select * from foo_view;

INSERT INTO foo_view(x, y, a, b, z)
  VALUES
  (5, null, 1, 2, true)
ON CONFLICT (x, a) where z=true
  DO UPDATE
  set b = EXCLUDED.b;

But, I get the exception:

ERROR:  there is no unique or exclusion constraint matching the ON CONFLICT specification

I can insert into foo instead of foo_view with the same ON CONFLICT without error.

Here is a fiddle: https://www.db-fiddle.com/f/cX2HXg91Q7yKoPeMBYzVLg/0