Bug hasn't been fully confirmed yet, but I suspect there is an issue with the recent changes to pg-connection-string.

I think f305419 (PR #2971) may have inadvertently changed the behaviour of connection string parsing, for strings of the form:

postgresql://myuser:mypass@localhost/mydb?host=/cloudsql/my-project:europe-west2:my-sql-instance

This style connection string I believe was previously recommended in the Google Cloud docs for connecting to Google Cloud SQL instances, because the host includes forward slashes (although might not not have been, and doesn't seem to be the case any more).