[GSB] Track all conformance constraint sources.

Move the storage for the protocols to which a particular potential
archetype conforms into EquivalenceClass, so that it is more easily
shared. More importantly, keep track of *all* of the constraint
sources that produced a particular conformance requirement, so we can
revisit them later, which provides a number of improvements:

* We can drop self-derived requirements at the end, once we've
  established all of the equivalence classes
* We diagnose redundant conformance requirements, e.g., "T: Sequence"
  is redundant if "T: Collection" is already specified.
* We can choose the best path when forming the conformance access
  path.

include/swift/AST/DiagnosticsSema.def

@@ -1571,6 +1571,12 @@ ERROR(requires_same_concrete_type,none,
 ERROR(protocol_typealias_conflict, none,
       "typealias %0 requires types %1 and %2 to be the same",
       (Identifier, Type, Type))
+WARNING(redundant_conformance_constraint,none,
+        "redundant conformance constraint %0: %1", (Type, ProtocolDecl *))
+NOTE(redundant_conformance_here,none,
+     "conformance constraint %1: %2 %select{written here|implied here}0",
+     (bool, Type, ProtocolDecl *))
+
 WARNING(redundant_same_type_to_concrete,none,
         "redundant same-type constraint %0 == %1", (Type, Type))
 NOTE(same_type_redundancy_here,none,

include/swift/AST/GenericSignatureBuilder.h

@@ -96,6 +96,14 @@ public:
 
   /// Describes an equivalence class of potential archetypes.
   struct EquivalenceClass {
+    /// The list of protocols to which this equivalence class conforms.
+    ///
+    /// The keys form the (semantic) list of protocols to which this type
+    /// conforms. The values are the conformance constraints as written on
+    /// this equivalence class.
+    llvm::MapVector<ProtocolDecl *, std::vector<Constraint<ProtocolDecl *>>>
+      conformsTo;
+
     /// Concrete type to which this equivalence class is equal.
     ///
     /// This is the semantic concrete type; the constraints as written
@@ -134,7 +142,11 @@ public:
     Optional<ConcreteConstraint>
     findAnySuperclassConstraintAsWritten(
                               PotentialArchetype *preferredPA = nullptr) const;
-};
+
+    /// Determine whether conformance to the given protocol is satisfied by
+    /// a superclass requirement.
+    bool isConformanceSatisfiedBySuperclass(ProtocolDecl *proto) const;
+  };
 
   friend class RequirementSource;
 
@@ -166,13 +178,9 @@ private:
   /// queried.
   ///
   /// \param proto The protocol to which we are establishing conformance.
-  ///
-  /// \param protoSource The requirement source for the conformance to the
-  /// given protocol.
   const RequirementSource *resolveSuperConformance(
                             GenericSignatureBuilder::PotentialArchetype *pa,
-                            ProtocolDecl *proto,
+                            ProtocolDecl *proto);
-                            const RequirementSource *&protoSource);
 
   /// \brief Add a new conformance requirement specifying that the given
   /// potential archetype conforms to the given protocol.
@@ -462,6 +470,12 @@ private:
                             ArrayRef<GenericTypeParamType *> genericParams,
                             PotentialArchetype *pa);
 
+  /// Check conformance constraints within the equivalence class of the
+  /// given potential archetype.
+  void checkConformanceConstraints(
+                            ArrayRef<GenericTypeParamType *> genericParams,
+                            PotentialArchetype *pa);
+
 public:
   /// \brief Resolve the given type to the potential archetype it names.
   ///
@@ -1016,9 +1030,6 @@ class GenericSignatureBuilder::PotentialArchetype {
   llvm::MapVector<PotentialArchetype *, const RequirementSource *>
     SameTypeConstraints;
 
-  /// \brief The list of protocols to which this archetype will conform.
-  llvm::MapVector<ProtocolDecl *, const RequirementSource *> ConformsTo;
-
   /// \brief The layout constraint of this archetype, if specified.
   LayoutConstraint Layout;
 
@@ -1215,16 +1226,23 @@ public:
     return dyn_cast<TypeAliasDecl>(identifier.assocTypeOrAlias);
   }
 
-  /// Retrieve the set of protocols to which this type conforms.
+  /// Retrieve the set of protocols to which this potential archetype
-  llvm::MapVector<ProtocolDecl *, const RequirementSource *> &
+  /// conforms.
-  getConformsTo() {
+  SmallVector<ProtocolDecl *, 4> getConformsTo() const {
-    return ConformsTo;
+    SmallVector<ProtocolDecl *, 4> result;
+
+    if (auto equiv = getEquivalenceClassIfPresent()) {
+      for (const auto &entry : equiv->conformsTo)
+        result.push_back(entry.first);
+    }
+
+    return result;
   }
 
   /// Add a conformance to this potential archetype.
   ///
   /// \returns true if the conformance was new, false if it already existed.
-  bool addConformance(ProtocolDecl *proto, bool updateExistingSource,
+  bool addConformance(ProtocolDecl *proto,
                       const RequirementSource *source,
                       GenericSignatureBuilder &builder);
 

lib/AST/GenericSignature.cpp

@@ -528,7 +528,7 @@ bool GenericSignature::requiresClass(Type type, ModuleDecl &mod) {
 
   // If any of the protocols are class-bound, then it must be a class.
   for (auto proto : pa->getConformsTo()) {
-    if (proto.first->requiresClass()) return true;
+    if (proto->requiresClass()) return true;
   }
 
   return false;
@@ -571,7 +571,7 @@ SmallVector<ProtocolDecl *, 2> GenericSignature::getConformsTo(Type type,
   // Retrieve the protocols to which this type conforms.
   SmallVector<ProtocolDecl *, 2> result;
   for (auto proto : pa->getConformsTo())
-    result.push_back(proto.first);
+    result.push_back(proto);
 
   // Canonicalize the resulting set of protocols.
   ProtocolType::canonicalizeProtocols(result);
@@ -738,6 +738,26 @@ static Type eraseAssociatedTypes(Type type) {
   return type;
 }
 
+namespace {
+  typedef GenericSignatureBuilder::RequirementSource RequirementSource;
+
+  template<typename T>
+  using GSBConstraint = GenericSignatureBuilder::Constraint<T>;
+}
+
+/// Retrieve the best requirement source from the list
+static const RequirementSource *
+getBestRequirementSource(ArrayRef<GSBConstraint<ProtocolDecl *>> constraints) {
+  const RequirementSource *bestSource = nullptr;
+  for (const auto &constraint : constraints) {
+    auto source = constraint.source;
+    if (!bestSource || source->compare(bestSource) < 0)
+      bestSource = source;
+  }
+
+  return bestSource;
+}
+
 ConformanceAccessPath GenericSignature::getConformanceAccessPath(
                                                        Type type,
                                                        ProtocolDecl *protocol,
@@ -747,12 +767,12 @@ ConformanceAccessPath GenericSignature::getConformanceAccessPath(
   // Resolve this type to a potential archetype.
   auto &builder = *getGenericSignatureBuilder(mod);
   auto pa = builder.resolveArchetype(type);
-  auto rep = pa->getRepresentative();
+  auto equivClass = pa->getOrCreateEquivalenceClass();
 
   // Dig out the conformance of this type to the given protocol, because we
   // want its requirement source.
-  auto conforms = rep->getConformsTo().find(protocol);
+  auto conforms = equivClass->conformsTo.find(protocol);
-  assert(conforms != rep->getConformsTo().end());
+  assert(conforms != equivClass->conformsTo.end());
 
   // Follow the requirement source to form the conformance access path.
   typedef GenericSignatureBuilder::RequirementSource RequirementSource;
@@ -837,16 +857,17 @@ ConformanceAccessPath GenericSignature::getConformanceAccessPath(
 
       // Dig out the potential archetype for this stored type.
       auto pa = reqSigBuilder.resolveArchetype(storedType);
-      auto rep = pa->getRepresentative();
+      auto equivClass = pa->getOrCreateEquivalenceClass();
 
       // Find the conformance of this potential archetype to the protocol in
       // question.
-      auto knownConforms = rep->getConformsTo().find(conformingProto);
+      auto conforms = equivClass->conformsTo.find(conformingProto);
-      assert(knownConforms != rep->getConformsTo().end());
+      assert(conforms != equivClass->conformsTo.end());
 
       // Compute the root type, canonicalizing it w.r.t. the protocol context.
       auto inProtoSig = inProtocol->getGenericSignature();
-      Type localRootType = knownConforms->second->getRootPotentialArchetype()
+      auto conformsSource = getBestRequirementSource(conforms->second);
+      Type localRootType = conformsSource->getRootPotentialArchetype()
                              ->getDependentType(inProtoSig->getGenericParams(),
                                                 /*allowUnresolved*/true);
       localRootType = inProtoSig->getCanonicalTypeInContext(
@@ -854,7 +875,7 @@ ConformanceAccessPath GenericSignature::getConformanceAccessPath(
                                                *inProtocol->getModuleContext());
 
       // Build the path according to the requirement signature.
-      buildPath(reqSig, knownConforms->second, conformingProto, localRootType);
+      buildPath(reqSig, conformsSource, conformingProto, localRootType);
 
       // We're done.
       return;
@@ -884,14 +905,14 @@ ConformanceAccessPath GenericSignature::getConformanceAccessPath(
   };
 
   // Canonicalize the root type.
-  auto source = conforms->second;
+  auto source = getBestRequirementSource(conforms->second);
   auto subjectPA = source->getRootPotentialArchetype();
   subjectPA = subjectPA->getArchetypeAnchor(*subjectPA->getBuilder());
   Type rootType = subjectPA->getDependentType(getGenericParams(),
                                               /*allowUnresolved=*/false);
 
   // Build the path.
-  buildPath(this, conforms->second, protocol, rootType);
+  buildPath(this, source, protocol, rootType);
 
   // Return the path; we're done!
   return path;

lib/AST/GenericSignatureBuilder.cpp

@@ -237,6 +237,11 @@ bool RequirementSource::isSelfDerivedSource(PotentialArchetype *pa) const {
     case RequirementSource::Inferred:
     case RequirementSource::RequirementSignatureSelf:
       currentPA = source->getRootPotentialArchetype();
+      while (auto parent = currentPA->getParent()) {
+        if (auto assocType = currentPA->getResolvedAssociatedType())
+          assocTypes.push_back(assocType);
+        currentPA = parent;
+      }
       break;
 
     case RequirementSource::Concrete:
@@ -262,7 +267,6 @@ bool RequirementSource::isSelfDerivedSource(PotentialArchetype *pa) const {
     currentPA = knownNested->second.front();
   }
 
-  // FIXME: currentPA == pa?
   return false;
 }
 
@@ -798,6 +802,18 @@ EquivalenceClass::findAnySuperclassConstraintAsWritten(
   return result;
 }
 
+bool EquivalenceClass::isConformanceSatisfiedBySuperclass(
+                                                    ProtocolDecl *proto) const {
+  auto known = conformsTo.find(proto);
+  assert(known != conformsTo.end() && "doesn't conform to this protocol");
+  for (const auto &constraint: known->second) {
+    if (constraint.source->kind == RequirementSource::Superclass)
+      return true;
+  }
+
+  return false;
+}
+
 bool GenericSignatureBuilder::updateRequirementSource(
                                       const RequirementSource *&existingSource,
                                       const RequirementSource *newSource) {
@@ -816,8 +832,7 @@ bool GenericSignatureBuilder::updateRequirementSource(
 
 const RequirementSource *GenericSignatureBuilder::resolveSuperConformance(
                       GenericSignatureBuilder::PotentialArchetype *pa,
-                      ProtocolDecl *proto,
+                      ProtocolDecl *proto) {
-                      const RequirementSource *&protoSource) {
   // Get the superclass constraint.
   Type superclass = pa->getSuperclass();
   if (!superclass) return nullptr;
@@ -843,8 +858,7 @@ const RequirementSource *GenericSignatureBuilder::resolveSuperConformance(
 
   superclassSource =
     superclassSource->viaSuperclass(*this, conformance->getConcrete());
-  if (protoSource)
+  paEquivClass->conformsTo[proto].push_back({pa, proto, superclassSource});
-    updateRequirementSource(protoSource, superclassSource);
   return superclassSource;
 }
 
@@ -926,29 +940,23 @@ static DeclRange getProtocolMembers(ProtocolDecl *proto) {
 }
 
 bool PotentialArchetype::addConformance(ProtocolDecl *proto,
-                                        bool updateExistingSource,
                                         const RequirementSource *source,
                                         GenericSignatureBuilder &builder) {
-  auto rep = getRepresentative();
+  // Check whether we already knew about this conformance.
-  if (rep != this)
+  auto equivClass = getOrCreateEquivalenceClass();
-    return rep->addConformance(proto, updateExistingSource, source, builder);
+  auto known = equivClass->conformsTo.find(proto);
-
+  if (known != equivClass->conformsTo.end()) {
-  // Check whether we already know about this conformance.
+    // We already knew about this conformance; record this specific constraint.
-  auto known = ConformsTo.find(proto);
+    known->second.push_back({this, proto, source});
-  if (known != ConformsTo.end()) {
-    // We already have this requirement. Update the requirement source
-    // appropriately.
-    if (updateExistingSource)
-      builder.updateRequirementSource(known->second, source);
     return false;
   }
 
-  // Add this conformance.
+  // Add the conformance along with this constraint.
-  auto inserted = ConformsTo.insert(std::make_pair(proto, source)).first;
+  equivClass->conformsTo[proto].push_back({this, proto, source});
 
-  // If there is a superclass, try to resolve the conformance immediately via
+  // Determine whether there is a superclass constraint where the
-  // the superclass.
+  // superclass conforms to this protocol.
-  getBuilder()->resolveSuperConformance(this, proto, inserted->second);
+  (void)getBuilder()->resolveSuperConformance(this, proto);
 
   // Resolve any existing nested types that need it.
   for (auto &nested : NestedTypes) {
@@ -1261,9 +1269,8 @@ PotentialArchetype *PotentialArchetype::getNestedArchetypeAnchor(
   TypeAliasDecl *bestTypeAlias = nullptr;
   SmallVector<TypeAliasDecl *, 4> typealiases;
   auto rep = getRepresentative();
-  for (const auto &conforms : rep->getConformsTo()) {
+  for (auto proto : rep->getConformsTo()) {
     // Look for an associated type and/or typealias with this name.
-    auto proto = conforms.first;
     AssociatedTypeDecl *assocType = nullptr;
     TypeAliasDecl *typealias = nullptr;
     for (auto member : proto->lookupDirect(name,
@@ -1494,21 +1501,8 @@ PotentialArchetype *PotentialArchetype::updateNestedTypeForConformance(
 
     // If there's a superclass constraint that conforms to the protocol,
     // add the appropriate same-type relationship.
-    auto rep = getRepresentative();
+    if (auto superSource = builder.resolveSuperConformance(this, proto))
-    auto knownConformance = rep->getConformsTo().find(proto);
-    if (knownConformance != rep->getConformsTo().end()) {
-      auto superSource =
-        builder.resolveSuperConformance(this, proto, knownConformance->second);
-
       maybeAddSameTypeRequirementForNestedType(resultPA, superSource, builder);
-    } else {
-      // FIXME: Dropping unknown conformance source on the floor.
-      const RequirementSource *nullSource = nullptr;
-      auto superSource = builder.resolveSuperConformance(this, proto,
-                                                         nullSource);
-
-      maybeAddSameTypeRequirementForNestedType(resultPA, superSource, builder);
-    }
 
     // We know something concrete about the parent PA, so we need to propagate
     // that information to this new archetype.
@@ -1551,6 +1545,7 @@ Type GenericSignatureBuilder::PotentialArchetype::getTypeInContext(
     return archetypeAnchor->getTypeInContext(builder, genericEnv);
 
   auto representative = getRepresentative();
+  auto equivClass = representative->getOrCreateEquivalenceClass();
   ASTContext &ctx = genericEnv->getGenericSignature()->getASTContext();
 
   // Return a concrete type or archetype we've already resolved.
@@ -1655,9 +1650,9 @@ Type GenericSignatureBuilder::PotentialArchetype::getTypeInContext(
 
   // Collect the protocol conformances for the archetype.
   SmallVector<ProtocolDecl *, 4> Protos;
-  for (const auto &conforms : representative->getConformsTo()) {
+  for (auto proto : representative->getConformsTo()) {
-    if (conforms.second->kind != RequirementSource::Superclass)
+    if (!equivClass || !equivClass->isConformanceSatisfiedBySuperclass(proto))
-      Protos.push_back(conforms.first);
+      Protos.push_back(proto);
   }
 
   // Create the archetype.
@@ -1791,20 +1786,27 @@ void GenericSignatureBuilder::PotentialArchetype::dump(llvm::raw_ostream &Out,
   }
 
   // Print requirements.
-  if (!ConformsTo.empty()) {
+  if (equivClass) {
-    Out << " : ";
+    bool First = true;
+    for (const auto &entry : equivClass->conformsTo) {
+      for (const auto &constraint : entry.second) {
+        if (constraint.archetype != this) continue;
 
-    interleave(ConformsTo,
+        if (First) {
-               [&](std::pair<ProtocolDecl *, const RequirementSource *>
+          First = false;
-                       ProtoAndSource) {
+          Out << ": ";
-                 Out << ProtoAndSource.first->getName().str() << " ";
+        } else {
-                 if (!ProtoAndSource.second->isDerivedRequirement())
+          Out << " & ";
-                   Out << "*";
+        }
-                 Out << "[";
+
-                 ProtoAndSource.second->print(Out, SrcMgr);
+        Out << constraint.value->getName().str() << " ";
-                 Out << "]";
+        if (!constraint.source->isDerivedRequirement())
-               },
+          Out << "*";
-               [&] { Out << " & "; });
+        Out << "[";
+        constraint.source->print(Out, SrcMgr);
+        Out << "]";
+      }
+    }
   }
 
   if (getRepresentative() != this) {
@@ -1998,13 +2000,13 @@ bool GenericSignatureBuilder::addConformanceRequirement(PotentialArchetype *PAT,
                                                  ProtocolDecl *Proto,
                                                  const RequirementSource *Source,
                                llvm::SmallPtrSetImpl<ProtocolDecl *> &Visited) {
+  // Add the requirement, if we haven't done so already.
+  if (!PAT->addConformance(Proto, Source, *this))
+    return false;
+
   // Add the requirement to the representative.
   auto T = PAT->getRepresentative();
 
-  // Add the requirement, if we haven't done so already.
-  if (!T->addConformance(Proto, /*updateExistingSource=*/true, Source, *this))
-    return false;
-
   bool inserted = Visited.insert(Proto).second;
   assert(inserted);
   (void) inserted;
@@ -2120,10 +2122,9 @@ bool GenericSignatureBuilder::updateSuperclass(
   // Local function to handle the update of superclass conformances
   // when the superclass constraint changes.
   auto updateSuperclassConformances = [&] {
-    for (auto &conforms : T->ConformsTo) {
+    for (auto proto : T->getConformsTo()) {
-      if (auto superSource = resolveSuperConformance(T, conforms.first,
+      if (auto superSource = resolveSuperConformance(T, proto)) {
-                                                     conforms.second)) {
+        for (auto req : getProtocolMembers(proto)) {
-        for (auto req : getProtocolMembers(conforms.first)) {
           auto assocType = dyn_cast<AssociatedTypeDecl>(req);
           if (!assocType) continue;
 
@@ -2242,24 +2243,6 @@ bool GenericSignatureBuilder::addSameTypeRequirementBetweenArchetypes(
     if (mismatch) return true;
   }
 
-  // Don't mark requirements as redundant if they come from one of our
-  // child archetypes. This is a targeted fix -- more general cases
-  // continue to break. In general, we need to detect cycles in the
-  // archetype graph and not propagate requirement source information
-  // along back edges.
-  bool updateExistingSource = true;
-  auto T2Parent = T2;
-  while (T2Parent != nullptr) {
-    if (T2Parent->getRepresentative() == T1)
-      updateExistingSource = false;
-    T2Parent = T2Parent->getParent();
-  }
-
-  // Another targeted fix -- don't drop conformances from generic
-  // parameters.
-  if (T1->getParent() == nullptr)
-    updateExistingSource = false;
-
   // Merge the equivalence classes.
   auto equivClass = T1->getOrCreateEquivalenceClass();
   auto equivClass2Members = T2->getEquivalenceClassMembers();
@@ -2304,9 +2287,15 @@ bool GenericSignatureBuilder::addSameTypeRequirementBetweenArchetypes(
   }
 
   // Add all of the protocol conformance requirements of T2 to T1.
-  for (auto conforms : T2->ConformsTo) {
+  if (equivClass2) {
-    T1->addConformance(conforms.first, updateExistingSource,
+    for (const auto &entry : equivClass2->conformsTo) {
-                       conforms.second, *this);
+      T1->addConformance(entry.first, entry.second.front().source, *this);
+
+      auto &constraints1 = equivClass->conformsTo[entry.first];
+      constraints1.insert(constraints1.end(),
+                          entry.second.begin() + 1,
+                          entry.second.end());
+    }
   }
 
   // Recursively merge the associated types of T2 into T1.
@@ -2356,8 +2345,7 @@ bool GenericSignatureBuilder::addSameTypeRequirementToConcrete(
   DenseMap<ProtocolDecl *, ProtocolConformanceRef> conformances;
   CanType depTy = rep->getDependentType({ }, /*allowUnresolved=*/true)
                     ->getCanonicalType();
-  for (auto &conforms : rep->getConformsTo()) {
+  for (auto protocol : rep->getConformsTo()) {
-    auto protocol = conforms.first;
     auto conformance =
       getLookupConformanceFn()(depTy, Concrete,
                                protocol->getDeclaredInterfaceType()
@@ -2380,7 +2368,7 @@ bool GenericSignatureBuilder::addSameTypeRequirementToConcrete(
                                               conformance->isConcrete()
                                                 ? conformance->getConcrete()
                                                 : nullptr);
-    updateRequirementSource(conforms.second, concreteSource);
+    equivClass->conformsTo[protocol].push_back({T, protocol, concreteSource});
   }
 
   // Eagerly resolve any existing nested types to their concrete forms (others
@@ -2493,8 +2481,7 @@ void GenericSignatureBuilder::markPotentialArchetypeRecursive(
     return;
   pa->setIsRecursive();
 
-  // FIXME: Drop this protocol.
+  pa->addConformance(proto, source, *this);
-  pa->addConformance(proto, /*updateExistingSource=*/true, source, *this);
   if (!pa->getParent())
     return;
 
@@ -2809,8 +2796,7 @@ static Identifier typoCorrectNestedType(
   llvm::SmallVector<Identifier, 2> bestMatches;
   unsigned bestEditDistance = 0;
   unsigned maxScore = (name.size() + 1) / 3;
-  for (const auto &conforms : pa->getParent()->getConformsTo()) {
+  for (auto proto : pa->getParent()->getConformsTo()) {
-    auto proto = conforms.first;
     for (auto member : getProtocolMembers(proto)) {
       auto assocType = dyn_cast<AssociatedTypeDecl>(member);
       if (!assocType)
@@ -2984,6 +2970,8 @@ GenericSignatureBuilder::finalize(SourceLoc loc,
         checkRedundantSuperclassConstraints(genericParams, archetype);
       }
     }
+
+    checkConformanceConstraints(genericParams, archetype);
   });
 
   // Check for generic parameters which have been made concrete or equated
@@ -3139,6 +3127,7 @@ Constraint<T> GenericSignatureBuilder::checkConstraintList(
                            Diag<Type, T> redundancyDiag,
                            Diag<bool, Type, T> otherNoteDiag) {
   // Remove self-derived constraints.
+  assert(!constraints.empty() && "No constraints?");
   constraints.erase(
     std::remove_if(constraints.begin(), constraints.end(),
                    [&](const Constraint<T> &constraint) {
@@ -3146,6 +3135,7 @@ Constraint<T> GenericSignatureBuilder::checkConstraintList(
                               constraint.archetype);
                    }),
     constraints.end());
+  assert(!constraints.empty() && "All constraints were self-derived!");
 
   // Sort the constraints, so we get a deterministic ordering of diagnostics.
   llvm::array_pod_sort(constraints.begin(), constraints.end());
@@ -3300,6 +3290,29 @@ Constraint<T> GenericSignatureBuilder::checkConstraintList(
   return *representativeConstraint;
 }
 
+void GenericSignatureBuilder::checkConformanceConstraints(
+                          ArrayRef<GenericTypeParamType *> genericParams,
+                          PotentialArchetype *pa) {
+  auto equivClass = pa->getEquivalenceClassIfPresent();
+  if (!equivClass || equivClass->conformsTo.empty())
+    return;
+
+  for (auto &entry : equivClass->conformsTo) {
+    checkConstraintList<ProtocolDecl *>(
+      genericParams, entry.second,
+      [](const Constraint<ProtocolDecl *> &constraint) {
+        return true;
+      },
+      [&](ProtocolDecl *proto) {
+        assert(proto == entry.first && "Mixed up protocol constraints");
+        return ConstraintRelation::Redundant;
+      },
+      None,
+      diag::redundant_conformance_constraint,
+      diag::redundant_conformance_here);
+  }
+}
+
 void GenericSignatureBuilder::checkRedundantConcreteTypeConstraints(
                                  ArrayRef<GenericTypeParamType *> genericParams,
                                  PotentialArchetype *representative) {
@@ -3720,11 +3733,23 @@ void GenericSignatureBuilder::enumerateRequirements(llvm::function_ref<
     // Enumerate conformance requirements.
     SmallVector<ProtocolDecl *, 4> protocols;
     DenseMap<ProtocolDecl *, const RequirementSource *> protocolSources;
-    for (const auto &conforms : rep->getConformsTo()) {
+    auto equivClass = rep->getEquivalenceClassIfPresent();
-      protocols.push_back(conforms.first);
+    if (equivClass) {
-      assert(protocolSources.count(conforms.first) == 0 && 
+      for (const auto &conforms : equivClass->conformsTo) {
-             "redundant protocol requirement?");
+        protocols.push_back(conforms.first);
-      protocolSources.insert({conforms.first, conforms.second});
+        assert(protocolSources.count(conforms.first) == 0 && 
+               "redundant protocol requirement?");
+
+        // Find the best source among the constraints that describe conformance
+        // to this protocol.
+        auto bestSource = conforms.second.front().source;
+        for (const auto &constraint : conforms.second) {
+          if (constraint.source->compare(bestSource) < 0)
+            bestSource = constraint.source;
+        }
+
+        protocolSources.insert({conforms.first, bestSource});
+      }
     }
 
     // Sort the protocols in canonical order.

test/Generics/associated_type_typo.swift

@@ -30,8 +30,9 @@ func typoAssoc3<T : P2, U : P2>()
   where U.AssocP2.assoc : P3,  T.AssocP2.assoc : P4,
         T.AssocP2 == U.AssocP2 {}
 
+// expected-error@+3{{'T.AssocP2' does not have a member type named 'assoc'; did you mean 'Assoc'?}}{{47-52=Assoc}}
 // expected-error@+2{{'T' does not have a member type named 'Assocp2'; did you mean 'AssocP2'?}}{{39-46=AssocP2}}
-// expected-error@+1{{'T.AssocP2' does not have a member type named 'assoc'; did you mean 'Assoc'?}}{{47-52=Assoc}}
+// expected-error@+1{{'T.AssocP2' does not have a member type named 'assoc'; did you mean 'Assoc'?}}
 func typoAssoc4<T : P2>(_: T) where T.Assocp2.assoc : P3 {}
 
 
@@ -39,7 +40,7 @@ func typoAssoc4<T : P2>(_: T) where T.Assocp2.assoc : P3 {}
 // CHECK-GENERIC-NEXT: Requirements:
 // CHECK-GENERIC-NEXT:   τ_0_0 : P2 [τ_0_0: Explicit @ {{.*}}:21]
 // CHECK-GENERIC-NEXT:   τ_0_0[.P2].AssocP2 : P1 [τ_0_0: Explicit @ {{.*}}:21 -> Protocol requirement (via Self.AssocP2 in P2)]
-// CHECK-GENERIC-NEXT:   τ_0_0[.P2].AssocP2[.P1].Assoc : P3 [τ_0_0[.P2].AssocP2.assoc: Explicit @ {{.*}}:53]
+// CHECK-GENERIC-NEXT:   τ_0_0[.P2].AssocP2[.P1].Assoc : P3 [τ_0_0[.P2].AssocP2[.P1].Assoc: Explicit @ {{.*}}:53]
 // CHECK-GENERIC-NEXT: Potential archetypes
 
 // <rdar://problem/19620340>

test/Generics/canonicalization.swift

@@ -12,4 +12,9 @@ protocol Q : P {
 }
 
 func f<T>(t: T) where T : P, T : Q, T.A : P0 { } // expected-note{{'f(t:)' previously declared here}}
-func f<T>(t: T) where T : Q, T : P, T.A : P0 { } // expected-error{{invalid redeclaration of 'f(t:)'}}
+// expected-warning@-1{{redundant conformance constraint 'T': 'P'}}
+// expected-note@-2{{conformance constraint 'T': 'P' implied here}}
+
+func f<T>(t: T) where T : Q, T : P, T.A : P0 { } // expected-error{{invalid redeclaration of 'f(t:)'}}
+// expected-warning@-1{{redundant conformance constraint 'T': 'P'}}
+// expected-note@-2{{conformance constraint 'T': 'P' implied here}}

test/Generics/conformance_access_path.swift

@@ -56,8 +56,8 @@ func testPaths3<V: P5>(_ v: V) {
 	acceptQ0(v.getAssocP3())
 }
 
-protocol P6Unordered: P5Unordered {
+protocol P6Unordered: P5Unordered { // expected-note{{conformance constraint 'Self.A': 'P0' implied here}}
-	associatedtype A: P0
+	associatedtype A: P0 // expected-warning{{redundant conformance constraint 'Self.A': 'P0'}}
 }
 
 protocol P5Unordered {

test/Generics/protocol_requirement_signatures.swift

@@ -57,8 +57,9 @@ protocol Q5: Q2, Q3, Q4 {}
 // CHECK-LABEL: .Q6@
 // CHECK-NEXT: Requirement signature: <Self where Self : Q2, Self : Q3, Self : Q4>
 // CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q2, τ_0_0 : Q3, τ_0_0 : Q4>
-protocol Q6: Q2, Q3, Q4 {
+protocol Q6: Q2, // expected-note{{conformance constraint 'Self.X': 'P1' implied here}}
-    associatedtype X: P1
+             Q3, Q4 {
+    associatedtype X: P1 // expected-warning{{redundant conformance constraint 'Self.X': 'P1'}}
 }
 
 // multiple inheritance with a new conformance

test/Generics/requirement_inference.swift

@@ -126,6 +126,8 @@ func inferSameType1<T, U>(_ x: Model_P3_P4_Eq<T, U>) { }
 // CHECK-NEXT:   τ_0_0[.P3].P3Assoc : P2 [τ_0_0: Explicit @ {{.*}}:25 -> Protocol requirement (via Self.P3Assoc in P3)]
 // CHECK-NEXT:   τ_0_0[.P3].P3Assoc == τ_0_1[.P4].P4Assoc [τ_0_0[.P3].P3Assoc: Explicit]
 func inferSameType2<T : P3, U : P4>(_: T, _: U) where U.P4Assoc : P2, T.P3Assoc == U.P4Assoc {}
+// expected-warning@-1{{redundant conformance constraint 'T.P3Assoc': 'P2'}}
+// expected-note@-2{{conformance constraint 'T.P3Assoc': 'P2' implied here}}
 
 // CHECK-LABEL: .inferSameType3@
 // CHECK-NEXT: Requirements:
@@ -149,8 +151,8 @@ protocol P7 : P6 {
 
 // CHECK-LABEL: P7.nestedSameType1()@
 // CHECK: Canonical generic signature: <τ_0_0 where τ_0_0 : P7, τ_0_0.AssocP6.Element : P6, τ_0_0.AssocP6.Element == τ_0_0.AssocP7.AssocP6.Element>
-extension P7 where AssocP6.Element : P6, 
+extension P7 where AssocP6.Element : P6, // expected-note{{conformance constraint 'Self.AssocP6.Element': 'P6' written here}}
-        AssocP7.AssocP6.Element : P6,
+        AssocP7.AssocP6.Element : P6, // expected-warning{{redundant conformance constraint 'Self.AssocP6.Element': 'P6'}}
         AssocP6.Element == AssocP7.AssocP6.Element {
   func nestedSameType1() { }
 }

test/Generics/superclass_constraint.swift

@@ -75,17 +75,23 @@ extension P2 where Self.T : C {
 
 // CHECK: superclassConformance1
 // CHECK: Requirements:
-// CHECK-NEXT: τ_0_0 : C [τ_0_0: Explicit @ {{.*}}:46]
+// CHECK-NEXT: τ_0_0 : C [τ_0_0: Explicit @ {{.*}}:11]
-// CHECK-NEXT: τ_0_0 : P3 [τ_0_0: Explicit @ {{.*}}:46 -> Superclass (C: P3)]
+// CHECK-NEXT: τ_0_0 : P3 [τ_0_0: Explicit @ {{.*}}:11 -> Superclass (C: P3)]
 // CHECK: Canonical generic signature: <τ_0_0 where τ_0_0 : C>
-func superclassConformance1<T>(t: T) where T : C, T : P3 {}
+func superclassConformance1<T>(t: T)
+  where T : C, // expected-note{{conformance constraint 'T': 'P3' implied here}}
+        T : P3 {} // expected-warning{{redundant conformance constraint 'T': 'P3'}}
+
+
 
 // CHECK: superclassConformance2
 // CHECK: Requirements:
-// CHECK-NEXT: τ_0_0 : C [τ_0_0: Explicit @ {{.*}}:46]
+// CHECK-NEXT: τ_0_0 : C [τ_0_0: Explicit @ {{.*}}:11]
-// CHECK-NEXT: τ_0_0 : P3 [τ_0_0: Explicit @ {{.*}}:46 -> Superclass (C: P3)]
+// CHECK-NEXT: τ_0_0 : P3 [τ_0_0: Explicit @ {{.*}}:11 -> Superclass (C: P3)]
 // CHECK: Canonical generic signature: <τ_0_0 where τ_0_0 : C>
-func superclassConformance2<T>(t: T) where T : C, T : P3 {}
+func superclassConformance2<T>(t: T)
+  where T : C, // expected-note{{conformance constraint 'T': 'P3' implied here}}
+   T : P3 {} // expected-warning{{redundant conformance constraint 'T': 'P3'}}
 
 protocol P4 { }
 
@@ -99,6 +105,8 @@ class C2 : C, P4 { }
 func superclassConformance3<T>(t: T) where T : C, T : P4, T : C2 {}
 // expected-warning@-1{{redundant superclass constraint 'T' : 'C'}}
 // expected-note@-2{{superclass constraint 'T' : 'C2' written here}}
+// expected-warning@-3{{redundant conformance constraint 'T': 'P4'}}
+// expected-note@-4{{conformance constraint 'T': 'P4' implied here}}
 
 protocol P5: A { } // expected-error{{non-class type 'P5' cannot inherit from class 'A'}}
 

test/Parse/deprecated_where.swift

@@ -1,13 +1,16 @@
 // RUN: %target-swift-frontend -typecheck -verify %s
 
+protocol Mashable { }
+protocol Womparable { }
+
 // FuncDecl: Choose 0
 func f1<T>(x: T) {}
 
 // FuncDecl: Choose 1
 // 1: Inherited constraint
-func f2<T: Hashable>(x: T) {} // no-warning
+func f2<T: Mashable>(x: T) {} // no-warning
 // 2: Non-trailing where
-func f3<T where T: Comparable>(x: T) {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{10-30=}} {{37-37= where T: Comparable}}
+func f3<T where T: Womparable>(x: T) {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{10-30=}} {{37-37= where T: Womparable}}
 // 3: Has return type
 func f4<T>(x: T) -> Int { return 2 } // no-warning
 // 4: Trailing where
@@ -15,29 +18,29 @@ func f5<T>(x: T) where T : Equatable {} // no-warning
 
 // FuncDecl: Choose 2
 // 1,2
-func f12<T: Hashable where T: Comparable>(x: T) {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{21-41=}} {{48-48= where T: Comparable}}
+func f12<T: Mashable where T: Womparable>(x: T) {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{21-41=}} {{48-48= where T: Womparable}}
 // 1,3
-func f13<T: Hashable>(x: T) -> Int { return 2 } // no-warning
+func f13<T: Mashable>(x: T) -> Int { return 2 } // no-warning
 // 1,4
-func f14<T: Hashable>(x: T) where T: Equatable {} // no-warning
+func f14<T: Mashable>(x: T) where T: Equatable {} // no-warning
 // 2,3
-func f23<T where T: Comparable>(x: T) -> Int { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{11-31=}} {{45-45= where T: Comparable}}
+func f23<T where T: Womparable>(x: T) -> Int { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{11-31=}} {{45-45= where T: Womparable}}
 // 2,4
-func f24<T where T: Comparable>(x: T) where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{11-31=}} {{39-44=where T: Comparable,}}
+func f24<T where T: Womparable>(x: T) where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{11-31=}} {{39-44=where T: Womparable,}}
 // 3,4
 func f34<T>(x: T) -> Int where T: Equatable { return 2 } // no-warning
 
 // FuncDecl: Choose 3
 // 1,2,3
-func f123<T: Hashable where T: Comparable>(x: T) -> Int { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{22-42=}} {{56-56= where T: Comparable}}
+func f123<T: Mashable where T: Womparable>(x: T) -> Int { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{22-42=}} {{56-56= where T: Womparable}}
 // 1,2,4
-func f124<T: Hashable where T: Comparable>(x: T) where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{22-42=}} {{50-55=where T: Comparable,}}
+func f124<T: Mashable where T: Womparable>(x: T) where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{22-42=}} {{50-55=where T: Womparable,}}
 // 2,3,4
-func f234<T where T: Comparable>(x: T) -> Int where T: Equatable { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{12-32=}} {{47-52=where T: Comparable,}}
+func f234<T where T: Womparable>(x: T) -> Int where T: Equatable { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{12-32=}} {{47-52=where T: Womparable,}}
 
 // FuncDecl: Choose 4
 // 1,2,3,4
-func f1234<T: Hashable where T: Comparable>(x: T) -> Int where T: Equatable { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{23-43=}} {{58-63=where T: Comparable,}}
+func f1234<T: Mashable where T: Womparable>(x: T) -> Int where T: Equatable { return 2 } // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{23-43=}} {{58-63=where T: Womparable,}}
 
 
 
@@ -46,23 +49,23 @@ struct S0<T> {}
 
 // NominalTypeDecl: Choose 1
 // 1: Inherited constraint
-struct S1<T: Hashable> {} // no-warning
+struct S1<T: Mashable> {} // no-warning
 // 2: Non-trailing where
-struct S2<T where T: Comparable> {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{12-32=}} {{33-33= where T: Comparable}}
+struct S2<T where T: Womparable> {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{12-32=}} {{33-33= where T: Womparable}}
 // 3: Trailing where
 struct S3<T> where T : Equatable {} // no-warning
 
 // NominalTypeDecl: Choose 2
 // 1,2
-struct S12<T: Hashable where T: Comparable> {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{23-43=}} {{44-44= where T: Comparable}}
+struct S12<T: Mashable where T: Womparable> {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{23-43=}} {{44-44= where T: Womparable}}
 // 1,3
-struct S13<T: Hashable> where T: Equatable {} // no-warning
+struct S13<T: Mashable> where T: Equatable {} // no-warning
 // 2,3
-struct S23<T where T: Comparable> where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{13-33=}} {{35-40=where T: Comparable,}}
+struct S23<T where T: Womparable> where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{13-33=}} {{35-40=where T: Womparable,}}
 
 // NominalTypeDecl: Choose 3
 // 1,2,3
-struct S123<T: Hashable where T: Comparable> where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{24-44=}} {{46-51=where T: Comparable,}}
+struct S123<T: Mashable where T: Womparable> where T: Equatable {} // expected-warning {{'where' clause next to generic parameters is deprecated and will be removed in the future version of Swift}} {{24-44=}} {{46-51=where T: Womparable,}}
 
 
 protocol ProtoA {}

test/decl/protocol/protocols.swift

@@ -82,6 +82,8 @@ protocol CircleStart : CircleEnd { func circle_start() } // expected-error 2{{ci
 // expected-note@-1{{protocol 'CircleStart' declared here}}
 protocol CircleEnd : CircleMiddle { func circle_end()} // expected-note{{protocol 'CircleEnd' declared here}}
 
+// expected-warning@+2{{redundant conformance constraint 'Self': 'CircleTrivial'}}
+// expected-note@+1{{conformance constraint 'Self': 'CircleTrivial' implied here}}
 protocol CircleEntry : CircleTrivial { }
 protocol CircleTrivial : CircleTrivial { } // expected-error 3{{circular protocol inheritance CircleTrivial}}
 

test/decl/protocol/recursive_requirement.swift

@@ -52,7 +52,8 @@ protocol P3 {
 
 protocol P4 : P3 {}
 
-protocol DeclaredP : P3, P4 {}
+protocol DeclaredP : P3, // expected-warning{{redundant conformance constraint 'Self': 'P3'}}
+P4 {} // expected-note{{conformance constraint 'Self': 'P3' implied here}}
 
 struct Y3 : DeclaredP {
 }
@@ -75,7 +76,13 @@ protocol Gamma {
   associatedtype Delta: Alpha // expected-error{{type may not reference itself as a requirement}}
 }
 
-struct Epsilon<T: Alpha, U: Gamma> where T.Beta == U, U.Delta == T {}
+// FIXME: Redundancy diagnostics are odd here.
+struct Epsilon<T: Alpha, // expected-note{{conformance constraint 'U': 'Gamma' implied here}}
+// expected-warning@-1{{redundant conformance constraint 'T': 'Alpha'}}
+               U: Gamma> // expected-warning{{redundant conformance constraint 'U': 'Gamma'}}
+// expected-note@-1{{conformance constraint 'T': 'Alpha' implied here}}
+  where T.Beta == U,
+        U.Delta == T {}
 
 // -----