There were several problems with it:
1) `isMoreSpecific` should return true if a == b. Otherwise
`isMoreSpecificThenAllOf` would never return true because it's always
invoked with a collection that contains the candidate. K1 behaves
similarly, `OverridingUtil.isMoreSpecific` returns true if a == b.
So in fact, "more" should be understood as "not less" here.
2) `transitivelyMostSpecificMember` in `selectMostSpecificMember` was
always equal to the first element, so `isMoreSpecific` was invoked
with incorrect arguments.
3) At the end of `selectMostSpecificMember`, we selected the first
candidate with the non-flexible return type, however only dynamic
type was considered. We need to check `isFlexible` via type system
instead.
#KT-66120 Fixed
Alexander Udalov