AST: Remove implicit conversion from Witness to ConcreteDeclRef

include/swift/AST/Witness.h

@@ -150,13 +150,6 @@ public:
   /// Determines whether there is a witness at all.
   explicit operator bool() const { return !storage.isNull(); }
 
-  /// Implicit conversion to the \c ConcreteDeclRef, which is used by a
-  /// number of clients.
-  ///
-  /// FIXME: We probably want this to go away eventually, because clients using
-  /// it will all need to be cognizant of the synthetic environment.
-  operator ConcreteDeclRef() const { return getDeclRef(); }
-
   /// Determine whether this witness requires any substitutions.
   bool requiresSubstitution() const { return storage.is<StoredWitness *>(); }
 

lib/AST/ConformanceLookupTable.cpp

@@ -1131,7 +1131,7 @@ ConformanceLookupTable::getSatisfiedProtocolRequirementsForMember(
 
       auto normal = conf->getRootNormalConformance();
       normal->forEachValueWitness(resolver, [&](ValueDecl *req,
-                                              ConcreteDeclRef witness) {
+                                                Witness witness) {
         if (witness.getDecl() == member)
           reqs.push_back(req);
       });

lib/Serialization/Serialization.cpp

@@ -1642,7 +1642,7 @@ void Serializer::writeDefaultWitnessTable(const ProtocolDecl *proto,
   unsigned abbrCode = abbrCodes[DefaultWitnessTableLayout::Code];
   for (auto member : proto->getMembers()) {
     if (auto *value = dyn_cast<ValueDecl>(member)) {
-      ConcreteDeclRef witness = proto->getDefaultWitness(value);
+      auto witness = proto->getDefaultWitness(value);
       if (!witness)
         continue;