Sema: Remove obsolete cycle-breaking hack

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -2837,7 +2837,6 @@ AllMembersRequest::evaluate(
 }
 
 bool TypeChecker::isPassThroughTypealias(TypeAliasDecl *typealias,
-                                         Type underlyingType,
                                          NominalTypeDecl *nominal) {
   // Pass-through only makes sense when the typealias refers to a nominal
   // type.
@@ -2872,7 +2871,8 @@ bool TypeChecker::isPassThroughTypealias(TypeAliasDecl *typealias,
   // If neither is generic at this level, we have a pass-through typealias.
   if (!typealias->isGeneric()) return true;
 
-  auto boundGenericType = underlyingType->getAs<BoundGenericType>();
+  auto boundGenericType = typealias->getUnderlyingType()
+      ->getAs<BoundGenericType>();
   if (!boundGenericType) return false;
 
   // If our arguments line up with our innermost generic parameters, it's
@@ -2919,24 +2919,13 @@ ExtendedTypeRequest::evaluate(Evaluator &eval, ExtensionDecl *ext) const {
   // Hack to allow extending a generic typealias.
   if (auto *unboundGeneric = extendedType->getAs<UnboundGenericType>()) {
     if (auto *aliasDecl = dyn_cast<TypeAliasDecl>(unboundGeneric->getDecl())) {
-      // Nested Hack to break cycles if this is called before validation has
-      // finished.
-      if (aliasDecl->hasInterfaceType()) {
-        auto extendedNominal =
-            aliasDecl->getDeclaredInterfaceType()->getAnyNominal();
-        if (extendedNominal)
-          return TypeChecker::isPassThroughTypealias(
-                     aliasDecl, aliasDecl->getUnderlyingType(), extendedNominal)
-                     ? extendedType
-                     : extendedNominal->getDeclaredType();
-      } else {
-        if (auto ty = aliasDecl->getStructuralType()
-                          ->getAs<NominalOrBoundGenericNominalType>())
-          return TypeChecker::isPassThroughTypealias(aliasDecl, ty,
-                                                     ty->getDecl())
-                     ? extendedType
-                     : ty->getDecl()->getDeclaredType();
-      }
+      auto extendedNominal =
+          aliasDecl->getUnderlyingType()->getAnyNominal();
+      if (extendedNominal)
+        return TypeChecker::isPassThroughTypealias(
+                   aliasDecl, extendedNominal)
+                   ? extendedType
+                   : extendedNominal->getDeclaredType();
     }
   }
 

lib/Sema/TypeCheckGeneric.cpp

@@ -530,8 +530,7 @@ static void collectAdditionalExtensionRequirements(
 
   // If we have a passthrough typealias, add the requirements from its
   // generic signature.
-  if (typealias && TypeChecker::isPassThroughTypealias(
-                       typealias, typealias->getUnderlyingType(), nominal)) {
+  if (typealias && TypeChecker::isPassThroughTypealias(typealias, nominal)) {
     for (auto req : typealias->getGenericSignature().getRequirements())
       sameTypeReqs.push_back(req);
   }

lib/Sema/TypeChecker.h

@@ -341,8 +341,7 @@ Type substMemberTypeWithBase(ModuleDecl *module, TypeDecl *member, Type baseTy,
 /// typealias GX2<A> = X<A, A>
 /// typealias GX3<A, B> = X<B, A>
 /// \endcode
-bool isPassThroughTypealias(TypeAliasDecl *typealias, Type underlyingType,
-                            NominalTypeDecl *nominal);
+bool isPassThroughTypealias(TypeAliasDecl *typealias, NominalTypeDecl *nominal);
 
 /// Determine whether one type is a subtype of another.
 ///