Reimplement ArchetypeType::getParent() in terms of interface type + environment

Rather that requiring parent archetypes to be stored at the time of
construction, map the base of the interface type through the
generic environment.

Author: DougGregor

include/swift/AST/Types.h

@@ -5470,6 +5470,10 @@ class ArchetypeType : public SubstitutableType,
   /// Register a nested type with the given name.
   void registerNestedType(Identifier name, Type nested);
 
+  /// Retrieve the parent of this archetype, or null if this is a
+  /// primary archetype.
+  ArchetypeType *getParent() const;
+
   /// Return the root archetype parent of this archetype.
   ArchetypeType *getRoot() const;
 
@@ -5696,25 +5700,17 @@ class NestedArchetypeType final : public ArchetypeType,
   friend TrailingObjects;
   friend ArchetypeType;
 
-  ArchetypeType *Parent;
-
 public:
   /// getNew - Create a new nested archetype with the given associated type.
   ///
   /// The ConformsTo array will be copied into the ASTContext by this routine.
   static CanTypeWrapper<NestedArchetypeType>
-  getNew(const ASTContext &Ctx, ArchetypeType *Parent,
+  getNew(const ASTContext &Ctx,
          DependentMemberType *InterfaceType,
          SmallVectorImpl<ProtocolDecl *> &ConformsTo,
          Type Superclass, LayoutConstraint Layout,
          GenericEnvironment *Environment);
 
-  /// Retrieve the parent of this archetype, or null if this is a
-  /// primary archetype.
-  ArchetypeType *getParent() const {
-    return Parent;
-  }
-  
   AssociatedTypeDecl *getAssocType() const;
 
   static bool classof(const TypeBase *T) {
@@ -5727,7 +5723,6 @@ class NestedArchetypeType final : public ArchetypeType,
 
 private:
   NestedArchetypeType(const ASTContext &Ctx,
-                     ArchetypeType *Parent,
                      Type InterfaceType,
                      ArrayRef<ProtocolDecl *> ConformsTo,
                      Type Superclass, LayoutConstraint Layout,

lib/AST/GenericEnvironment.cpp

@@ -279,7 +279,7 @@ GenericEnvironment::getOrCreateArchetypeFromInterfaceType(Type depType) {
 
   if (parentArchetype) {
     auto *depMemTy = requirements.anchor->castTo<DependentMemberType>();
-    result = NestedArchetypeType::getNew(ctx, parentArchetype, depMemTy,
+    result = NestedArchetypeType::getNew(ctx, depMemTy,
                                          requirements.protos, superclass,
                                          requirements.layout, this);
     parentArchetype->registerNestedType(depMemTy->getName(), result);

lib/AST/Type.cpp

@@ -3210,6 +3210,15 @@ ArchetypeType::ArchetypeType(TypeKind Kind,
                           getSubclassTrailingObjects<ProtocolDecl *>());
 }
 
+ArchetypeType *ArchetypeType::getParent() const {
+  if (auto depMemTy = getInterfaceType()->getAs<DependentMemberType>()) {
+    return getGenericEnvironment()->mapTypeIntoContext(depMemTy->getBase())
+        ->castTo<ArchetypeType>();
+  }
+
+  return nullptr;
+}
+
 ArchetypeType *ArchetypeType::getRoot() const {
   auto gp = InterfaceType->getRootGenericParam();
   assert(gp && "Missing root generic parameter?");
@@ -3265,16 +3274,16 @@ OpenedArchetypeType::OpenedArchetypeType(const ASTContext &Ctx,
 }
 
 NestedArchetypeType::NestedArchetypeType(const ASTContext &Ctx,
-                                       ArchetypeType *Parent,
                                        Type InterfaceType,
                                        ArrayRef<ProtocolDecl *> ConformsTo,
                                        Type Superclass,
                                        LayoutConstraint Layout,
                                        GenericEnvironment *Environment)
   : ArchetypeType(TypeKind::NestedArchetype, Ctx,
-                  Parent->getRecursiveProperties(),
-                  InterfaceType, ConformsTo, Superclass, Layout, Environment),
-    Parent(Parent)
+                  Environment->mapTypeIntoContext(
+                      InterfaceType->getRootGenericParam())
+                        ->getRecursiveProperties(),
+                  InterfaceType, ConformsTo, Superclass, Layout, Environment)
 {
 }
 
@@ -3590,7 +3599,6 @@ operator()(CanType maybeOpaqueType, Type replacementType,
 
 CanNestedArchetypeType NestedArchetypeType::getNew(
                                    const ASTContext &Ctx,
-                                   ArchetypeType *Parent,
                                    DependentMemberType *InterfaceType,
                                    SmallVectorImpl<ProtocolDecl *> &ConformsTo,
                                    Type Superclass,
@@ -3608,7 +3616,7 @@ CanNestedArchetypeType NestedArchetypeType::getNew(
       alignof(NestedArchetypeType), arena);
 
   return CanNestedArchetypeType(::new (mem) NestedArchetypeType(
-      Ctx, Parent, InterfaceType, ConformsTo, Superclass, Layout, Environment));
+      Ctx, InterfaceType, ConformsTo, Superclass, Layout, Environment));
 }
 
 CanPrimaryArchetypeType