[AST] Record default associated conformance witnesses directly.

Referencing associated conformance witnesses via the index of the
requirement in the requirement signature is inconvenient. Reference
them by CanType/ProtocolDecl* instead.

Author: DougGregor

include/swift/AST/Decl.h

@@ -3981,19 +3981,15 @@ class ProtocolDecl final : public NominalTypeDecl {
 
   /// Returns the default associated conformance witness for an associated
   /// type, or \c None if there is no default.
-  ///
-  /// \param reqIndex The index of the associated conformance within the
-  /// requirement signature.
   Optional<ProtocolConformanceRef> getDefaultAssociatedConformanceWitness(
-                                                      unsigned reqIndex) const;
+                                              CanType association,
+                                              ProtocolDecl *requirement) const;
 
   /// Set the default associated conformance witness for the given
   /// associated conformance.
-  ///
-  /// \param reqIndex The index of the associated conformance within the
-  /// requirement signature.
   void setDefaultAssociatedConformanceWitness(
-                                            unsigned reqIndex,
+                                            CanType association,
+                                            ProtocolDecl *requirement,
                                             ProtocolConformanceRef conformance);
 
   /// Retrieve the name to use for this protocol when interoperating

lib/AST/ASTContext.cpp

@@ -286,7 +286,7 @@ FOR_KNOWN_FOUNDATION_TYPES(CACHE_FOUNDATION_DECL)
     DefaultTypeWitnesses;
 
   /// Default associated conformance witnesses for protocols.
-  llvm::DenseMap<std::pair<const ProtocolDecl *, unsigned>,
+  llvm::DenseMap<std::tuple<const ProtocolDecl *, CanType, ProtocolDecl *>,
                  ProtocolConformanceRef>
     DefaultAssociatedConformanceWitnesses;
 
@@ -1715,22 +1715,27 @@ void ProtocolDecl::setDefaultTypeWitness(AssociatedTypeDecl *assocType,
 }
 
 Optional<ProtocolConformanceRef>
-ProtocolDecl::getDefaultAssociatedConformanceWitness(unsigned reqIndex) const {
+ProtocolDecl::getDefaultAssociatedConformanceWitness(
+                                             CanType association,
+                                             ProtocolDecl *requirement) const {
   auto &ctx = getASTContext();
   auto found =
-    ctx.getImpl().DefaultAssociatedConformanceWitnesses.find({this, reqIndex});
+    ctx.getImpl().DefaultAssociatedConformanceWitnesses.find(
+      std::make_tuple(this, association, requirement));
   if (found == ctx.getImpl().DefaultAssociatedConformanceWitnesses.end())
     return None;
 
   return found->second;
 }
 
 void ProtocolDecl::setDefaultAssociatedConformanceWitness(
-                                          unsigned reqIndex,
+                                          CanType association,
+                                          ProtocolDecl *requirement,
                                           ProtocolConformanceRef conformance) {
   auto &ctx = getASTContext();
   auto pair = ctx.getImpl().DefaultAssociatedConformanceWitnesses.insert(
-                std::make_pair(std::make_pair(this, reqIndex), conformance));
+                std::make_pair(std::make_tuple(this, association, requirement),
+                               conformance));
   assert(pair.second && "Already have a default associated conformance");
   (void)pair;
 }

lib/Sema/TypeCheckProtocol.cpp

@@ -5304,9 +5304,7 @@ void TypeChecker::inferDefaultWitnesses(ProtocolDecl *proto) {
 
   // Find defaults for any associated conformances rooted on defaulted
   // associated types.
-  auto requirementSignature = proto->getRequirementSignature();
-  for (unsigned reqIndex : indices(requirementSignature)) {
-    const auto &req = requirementSignature[reqIndex];
+  for (const auto &req : proto->getRequirementSignature()) {
     if (req.getKind() != RequirementKind::Conformance)
       continue;
     if (req.getFirstType()->isEqual(proto->getProtocolSelfType()))
@@ -5371,7 +5369,8 @@ void TypeChecker::inferDefaultWitnesses(ProtocolDecl *proto) {
     }
 
     // Record the default associated conformance.
-    proto->setDefaultAssociatedConformanceWitness(reqIndex, *conformance);
+    proto->setDefaultAssociatedConformanceWitness(
+        req.getFirstType()->getCanonicalType(), requirementProto, *conformance);
   }
 }