[CSRanking] Account for that fact that some bindings can be type holes

Author: xedin

lib/Sema/CSRanking.cpp

@@ -1080,6 +1080,21 @@ SolutionCompareResult ConstraintSystem::compareSolutions(
       continue;
     }
 
+    // With introduction of holes it's currently possible to form solutions
+    // with UnresolvedType bindings, we need to account for that in
+    // ranking. If one solution has a hole for a given type variable
+    // it's always worse than any non-hole type other solution might have.
+    if (type1->is<UnresolvedType>() || type2->is<UnresolvedType>()) {
+      if (type1->is<UnresolvedType>()) {
+        ++score2;
+      } else {
+        ++score1;
+      }
+
+      identical = false;
+      continue;
+    }
+
     // If one type is a subtype of the other, but not vice-versa,
     // we prefer the system with the more-constrained type.
     // FIXME: Collapse this check into the second check.