[CSBindings] Reset literal coverage when retracting constraint

Author: xedin

include/swift/Sema/CSBindings.h

@@ -206,6 +206,11 @@ struct LiteralRequirement {
                                     bool canBeNil,
                                     DeclContext *useDC) const;
 
+  void resetCoverage() {
+    assert(isCovered() && "literal requirement is uncovered");
+    CoveredBy = nullptr;
+  }
+
   /// Determines whether literal protocol associated with this
   /// meta-information is viable for inclusion as a defaultable binding.
   bool viableAsBinding() const { return !isCovered() && hasDefaultType(); }

lib/Sema/CSBindings.cpp

@@ -1298,6 +1298,11 @@ void PotentialBindings::retract(Constraint *constraint) {
       AdjacentVars.erase(std::make_pair(adjacentVar, constraint));
   }
 
+  for (auto &literal : Literals) {
+    if (literal.second.CoveredBy == constraint)
+      literal.second.resetCoverage();
+  }
+
   DelayedBy.erase(llvm::remove_if(DelayedBy, isMatchingConstraint),
                   DelayedBy.end());