Remove unnecessary uses of NestedArchetypeType.

`ArchetypeType` has all of the API we need for these cases, so use
that instead to isolate us from `NestedArchetypeType`, which we would
like to eliminate soon-ish.

Author: DougGregor

lib/AST/Type.cpp

@@ -3409,7 +3409,7 @@ static bool canSubstituteTypeInto(Type ty, const DeclContext *dc,
     // The referenced type might be a different opaque result type.
 
     // First, unwrap any nested associated types to get the root archetype.
-    if (auto nestedTy = ty->getAs<NestedArchetypeType>())
+    if (auto nestedTy = ty->getAs<ArchetypeType>())
       ty = nestedTy->getRoot();
 
     // If the root archetype is an opaque result type, check that its
@@ -4258,8 +4258,9 @@ static Type substType(Type derivedType,
       return Type(type);
 
     // For nested archetypes, we can substitute the parent.
-    auto nestedArchetype = cast<NestedArchetypeType>(substOrig);
+    auto nestedArchetype = cast<ArchetypeType>(substOrig);
     auto parent = nestedArchetype->getParent();
+    assert(parent && "Not a nested archetype");
 
     // Substitute into the parent type.
     Type substParent = substType(parent, substitutions,
@@ -4270,11 +4271,11 @@ static Type substType(Type derivedType,
       return Type(type);
 
     // Get the associated type reference from a child archetype.
-    AssociatedTypeDecl *assocType = nestedArchetype->getAssocType();
+    AssociatedTypeDecl *assocType = nestedArchetype->getInterfaceType()
+        ->castTo<DependentMemberType>()->getAssocType();
 
     return getMemberForBaseType(lookupConformances, parent, substParent,
-                                assocType, nestedArchetype->getName(),
-                                options);
+                                assocType, assocType->getName(), options);
   });
 }
 

lib/IRGen/GenArchetype.cpp

@@ -70,10 +70,12 @@ irgen::emitArchetypeTypeMetadataRef(IRGenFunction &IGF,
   }
 
   // If there's no local or opaque metadata, it must be a nested type.
-  auto nested = cast<NestedArchetypeType>(archetype);
+  assert(archetype->getParent() && "Not a nested archetype");
 
-  CanArchetypeType parent(nested->getParent());
-  AssociatedType association(nested->getAssocType());
+  CanArchetypeType parent(archetype->getParent());
+  AssociatedType association(
+      archetype->getInterfaceType()->castTo<DependentMemberType>()
+        ->getAssocType());
 
   MetadataResponse response =
     emitAssociatedTypeMetadataRef(IGF, parent, association, request);

lib/IRGen/GenReflection.cpp

@@ -199,8 +199,9 @@ getRuntimeVersionThatSupportsDemanglingType(CanType type) {
   // Swift 5.1 runtime, so we needed to add a new mangling in 5.2.
   if (type->hasOpaqueArchetype()) {
     auto hasOpaqueAssocType = type.findIf([](CanType t) -> bool {
-      if (auto a = dyn_cast<NestedArchetypeType>(t)) {
-        return isa<OpaqueTypeArchetypeType>(a->getRoot());
+      if (auto a = dyn_cast<ArchetypeType>(t)) {
+        return isa<OpaqueTypeArchetypeType>(a->getRoot()) &&
+          a->getInterfaceType()->is<DependentMemberType>();
       }
       return false;
     });

lib/IRGen/IRGenDebugInfo.cpp

@@ -1549,8 +1549,9 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
     case TypeKind::SequenceArchetype: {
       auto *Archetype = BaseTy->castTo<ArchetypeType>();
       AssociatedTypeDecl *assocType = nullptr;
-      if (auto nested = dyn_cast<NestedArchetypeType>(Archetype))
-        assocType = nested->getAssocType();
+      if (auto depMemTy = Archetype->getInterfaceType()
+              ->getAs<DependentMemberType>())
+        assocType = depMemTy->getAssocType();
       auto L = getFilenameAndLocation(*this, assocType);
       auto *File = getOrCreateFile(L.filename);
       unsigned FwdDeclLine = 0;