[CSStep] Make sure disjunction choices with value conversions are not considered "the best"

Augment `DisjunctionChoice::{shouldSkip, shouldStopAt}` to check
for presence of implicit value conversions, and if score indicates
that there is at least one of those was attempted - downgrade
importance of a last successful choice.

Author: xedin

lib/Sema/CSStep.cpp

@@ -710,6 +710,7 @@ bool DisjunctionStep::shouldStopAt(const DisjunctionChoice &choice) const {
   auto *lastChoice = LastSolvedChoice->first;
   auto delta = LastSolvedChoice->second - getCurrentScore();
   bool hasUnavailableOverloads = delta.Data[SK_Unavailable] > 0;
+  bool hasImplicitConversions = delta.Data[SK_ImplicitValueConversion] > 0;
   bool hasFixes = delta.Data[SK_Fix] > 0;
   bool hasAsyncMismatch = delta.Data[SK_AsyncInSyncMismatch] > 0;
   auto isBeginningOfPartition = choice.isBeginningOfPartition();
@@ -719,7 +720,9 @@ bool DisjunctionStep::shouldStopAt(const DisjunctionChoice &choice) const {
   //   1. selecting unavailable overloads
   //   2. result in fixes being applied to reach a solution
   //   3. selecting an overload that results in an async/sync mismatch
+  //   4. implicit value conversions
   return !hasUnavailableOverloads && !hasFixes && !hasAsyncMismatch &&
+         !hasImplicitConversions &&
          (isBeginningOfPartition ||
           shortCircuitDisjunctionAt(choice, lastChoice));
 }

lib/Sema/CSStep.h

@@ -691,10 +691,11 @@ class DisjunctionStep final : public BindingStep<DisjunctionChoiceProducer> {
     // non-generic score indicates that there were no forced
     // unwrappings of optional(s), no unavailable overload
     // choices present in the solution, no fixes required,
-    // and there are no non-trivial function conversions.
+    // and there are no non-trivial function or value conversions.
     auto &score = BestNonGenericScore->Data;
     return (score[SK_ForceUnchecked] == 0 && score[SK_Unavailable] == 0 &&
-            score[SK_Fix] == 0 && score[SK_FunctionConversion] == 0);
+            score[SK_Fix] == 0 && score[SK_FunctionConversion] == 0 &&
+            score[SK_ImplicitValueConversion] == 0);
   }
 
   /// Attempt to apply given disjunction choice to constraint system.