GSB: Record the original type for self-derived conformances

include/swift/AST/GenericSignatureBuilder.h

@@ -1118,18 +1118,24 @@ class GenericSignatureBuilder::RequirementSource final
                              WrittenRequirementLoc writtenLoc =
                                WrittenRequirementLoc()) const;
 public:
-  /// A requirement source that describes that a requirement that is resolved
+  /// A requirement source that describes a conformance requirement resolved
   /// via a superclass requirement.
   const RequirementSource *viaSuperclass(
                                     GenericSignatureBuilder &builder,
                                     ProtocolConformanceRef conformance) const;
 
-  /// A requirement source that describes that a requirement that is resolved
-  /// via a same-type-to-concrete requirement.
+  /// A requirement source that describes a conformance requirement resolved
+  /// via a concrete type requirement with a conforming nominal type.
   const RequirementSource *viaConcrete(
                                      GenericSignatureBuilder &builder,
                                      ProtocolConformanceRef conformance) const;
 
+  /// A requirement source that describes that a requirement that is resolved
+  /// via a concrete type requirement with an existential self-conforming type.
+  const RequirementSource *viaConcrete(
+                                     GenericSignatureBuilder &builder,
+                                     Type existentialType) const;
+
   /// A constraint source that describes a layout constraint that was implied
   /// by a superclass requirement.
   const RequirementSource *viaLayout(GenericSignatureBuilder &builder,

lib/AST/GenericSignatureBuilder.cpp

@@ -591,10 +591,10 @@ bool RequirementSource::isAcceptableStorageKind(Kind kind,
   case Superclass:
   case Concrete:
     switch (storageKind) {
+    case StorageKind::StoredType:
     case StorageKind::ProtocolConformance:
       return true;
 
-    case StorageKind::StoredType:
     case StorageKind::AssociatedTypeDecl:
     case StorageKind::None:
       return false;
@@ -1043,6 +1043,17 @@ const RequirementSource *RequirementSource::viaConcrete(
                         0, WrittenRequirementLoc());
 }
 
+const RequirementSource *RequirementSource::viaConcrete(
+                                    GenericSignatureBuilder &builder,
+                                    Type existentialType) const {
+  assert(existentialType->isExistentialType());
+  REQUIREMENT_SOURCE_FACTORY_BODY(
+                        (nodeID, Concrete, this, existentialType.getPointer(),
+                         nullptr, nullptr),
+                        (Concrete, this, existentialType),
+                        0, WrittenRequirementLoc());
+}
+
 const RequirementSource *RequirementSource::viaParent(
                                       GenericSignatureBuilder &builder,
                                       AssociatedTypeDecl *assocType) const {
@@ -1115,8 +1126,14 @@ const RequirementSource *RequirementSource::withoutRedundantSubpath(
                                getWrittenRequirementLoc());
 
   case Concrete:
-    return parent->withoutRedundantSubpath(builder, start, end)
-      ->viaConcrete(builder, getProtocolConformance());
+    if (auto existentialType = getStoredType()) {
+      assert(existentialType->isExistentialType());
+      return parent->withoutRedundantSubpath(builder, start, end)
+        ->viaConcrete(builder, existentialType);
+    } else {
+      return parent->withoutRedundantSubpath(builder, start, end)
+        ->viaConcrete(builder, getProtocolConformance());
+    }
 
   case Layout:
     return parent->withoutRedundantSubpath(builder, start, end)
@@ -2307,11 +2324,30 @@ GenericSignatureBuilder::resolveConcreteConformance(ResolvedType type,
     return nullptr;
   }
 
-  concreteSource = concreteSource->viaConcrete(*this, conformance);
-  equivClass->recordConformanceConstraint(*this, type, proto, concreteSource);
-  if (addConditionalRequirements(conformance, /*inferForModule=*/nullptr,
-                                 concreteSource->getLoc()))
-    return nullptr;
+  if (concrete->isExistentialType()) {
+    // If we have an existential type, record the original type, and
+    // not the conformance.
+    //
+    // The conformance must be a self-conformance, and self-conformances
+    // do not record the original type in the case where a derived
+    // protocol self-conforms to a base protocol; for example:
+    //
+    // @objc protocol Base {}
+    //
+    // @objc protocol Derived {}
+    //
+    // struct S<T : Base> {}
+    //
+    // extension S where T == Derived {}
+    assert(isa<SelfProtocolConformance>(conformance.getConcrete()));
+    concreteSource = concreteSource->viaConcrete(*this, concrete);
+  } else {
+    concreteSource = concreteSource->viaConcrete(*this, conformance);
+    equivClass->recordConformanceConstraint(*this, type, proto, concreteSource);
+    if (addConditionalRequirements(conformance, /*inferForModule=*/nullptr,
+                                   concreteSource->getLoc()))
+      return nullptr;
+  }
 
   return concreteSource;
 }
@@ -2329,6 +2365,8 @@ const RequirementSource *GenericSignatureBuilder::resolveSuperConformance(
   if (conformance.isInvalid())
     return nullptr;
 
+  assert(!conformance.isAbstract());
+
   // Conformance to this protocol is redundant; update the requirement source
   // appropriately.
   const RequirementSource *superclassSource;
@@ -2526,10 +2564,11 @@ static void concretizeNestedTypeFromConcreteParent(
       conformance.getConcrete()->getTypeWitness(assocType);
     if (!witnessType)
       return; // FIXME: should we delay here?
-  } else if (auto archetype = concreteParent->getAs<ArchetypeType>()) {
-    witnessType = archetype->getNestedType(assocType->getName());
   } else {
-    witnessType = DependentMemberType::get(concreteParent, assocType);
+    // Otherwise we have an abstract conformance to an opaque result type.
+    assert(conformance.isAbstract());
+    auto archetype = concreteParent->castTo<ArchetypeType>();
+    witnessType = archetype->getNestedType(assocType->getName());
   }
 
   builder.addSameTypeRequirement(
@@ -2549,8 +2588,7 @@ PotentialArchetype *PotentialArchetype::updateNestedTypeForConformance(
 
   Identifier name = assocType->getName();
 
-  // Look for either an unresolved potential archetype (which we can resolve
-  // now) or a potential archetype with the appropriate associated type.
+  // Look for a potential archetype with the appropriate associated type.
   PotentialArchetype *resultPA = nullptr;
   auto knownNestedTypes = NestedTypes.find(name);
   bool shouldUpdatePA = false;