A Universe of Sorts

Its like 5 steps
Find all instances I that match the target constraint; that is, the target constraint is a substitution instance of I. These instance declarations are the candidates.
If no candidates remain, the search fails. Eliminate any candidate IX
for which there is another candidate IY such that both of the following hold: IY is strictly more specific than IX. That is, IY is a substitution instance of IX but not vice versa. Either IX is overlappable, or IY is overlapping. (This “either/or” design, rather than a “both/and” design, allow a client to deliberately override an instance from a library, without requiring a change to the library.)
If all the remaining candidates are incoherent, the search succeeds, returning an arbitrary surviving candidate.
If more than one non-incoherent candidate remains, the search fails.
Otherwise there is exactly one non-incoherent candidate; call it the “prime candidate”.
Now find all instances, or in-scope given constraints, that unify with the target constraint, but do not match it. Such non-candidate instances might match when the target constraint is further instantiated. If all of them are incoherent top-level instances, the search succeeds, returning the prime candidate. Otherwise the search fails.
GHC manual

§ How GHC Does Typeclass Resolution