Re: Why password authentication failed for user "postgres"?

This is simply wrong (though I suppose only in a corner case).  You must connect via socket but peer authentication can still work.  In particular, so long as the value specified for "-U" is your operating system user name the connection will work just the same as if you didn't specify -U at all and instead relied on the psql default behavior of using your operating system user name for the value of user.  In short, the server only knows what value "user" has as part of the connection string - it has no knowledge of how that value became set.  However, it can prove that the socket connection being requested is owned by a particular user.