How do I upsert depending on a second table?

Attempt:
```sql
CREATE TABLE org
(
"name" VARCHAR(50) PRIMARY KEY,
owner VARCHAR(50) NOT NULL
);

CREATE TABLE repo
(
"id" INTEGER PRIMARY KEY,
full_name VARCHAR(255) UNIQUE NOT NULL,
org VARCHAR(50) NOT NULL REFERENCES org ("name")
);

INSERT INTO org(name, owner) VALUES ('org0', 'user0');

INSERT INTO repo (id, full_name, org)
VALUES (0, 'org0/name0 by wrong user', 'org0')
ON CONFLICT (full_name) DO UPDATE
SET full_name = EXCLUDED.full_name,
org = EXCLUDED.org
WHERE EXISTS (SELECT 1
FROM org org_tbl
WHERE org_tbl.name = EXCLUDED.org
AND org_tbl.owner = 'wrong user')
RETURNING *;

SELECT * FROM repo WHERE id = 0;
```

This all succeeds. It should fail because the 'wrong user' is trying
to create a new—or update an existing—repo.

Thanks for all suggestions