[Type checker] Update for 5.0 branch.

Author: DougGregor

lib/Sema/TypeCheckDecl.cpp

@@ -4973,7 +4973,7 @@ checkExtensionGenericParams(TypeChecker &tc, ExtensionDecl *ext, Type type,
 static bool isNonGenericTypeAliasType(Type type) {
   // A non-generic typealias can extend a specialized type.
   if (auto *aliasType = dyn_cast<NameAliasType>(type.getPointer()))
-    return aliasType->getDecl()->getGenericContextDepth() == (unsigned)-1;
+    return aliasType->getDecl()->getGenericParamsOfContext() == nullptr;
 
   return false;
 }