AST: TypeDecl::getDeclaredInterfaceType() always returns a non-empty Type

In the case of circular type aliases, this now returns ErrorType
instead of Type().

Author: slavapestov

lib/AST/Decl.cpp

@@ -3248,13 +3248,10 @@ Type TypeDecl::getDeclaredInterfaceType() const {
   }
 
   Type interfaceType = getInterfaceType();
-  if (interfaceType.isNull() || interfaceType->is<ErrorType>())
-    return interfaceType;
-
-  if (isa<ModuleDecl>(this))
-    return interfaceType;
+  if (!interfaceType)
+    return ErrorType::get(getASTContext());
 
-  return interfaceType->castTo<MetatypeType>()->getInstanceType();
+  return interfaceType->getMetatypeInstanceType();
 }
 
 int TypeDecl::compare(const TypeDecl *type1, const TypeDecl *type2) {

lib/AST/GenericSignatureBuilder.cpp

@@ -3811,8 +3811,6 @@ static Type substituteConcreteType(GenericSignatureBuilder &builder,
   // Form an unsubstituted type referring to the given type declaration,
   // for use in an inferred same-type requirement.
   auto type = getStructuralType(concreteDecl, /*keepSugar=*/true);
-  if (!type)
-    return Type();
 
   SubstitutionMap subMap;
   if (proto) {
@@ -4209,10 +4207,7 @@ ConstraintResult GenericSignatureBuilder::expandConformanceRequirement(
   // within this protocol or a protocol it inherits.
   auto addInferredSameTypeReq = [&](TypeDecl *first, TypeDecl *second) {
     Type firstType = getStructuralType(first, /*keepSugar=*/false);
-    if (!firstType) return;
-
     Type secondType = getStructuralType(second, /*keepSugar=*/false);
-    if (!secondType) return;
 
     auto inferredSameTypeSource =
       FloatingRequirementSource::viaProtocolRequirement(