Fix C++20 build

Summary: Fix C++20 build

Reviewed By: luciang

Differential Revision: D31786941

fbshipit-source-id: 153ffcee7b64f4c6dfafb786df90e909cee723b3

folly/gen/Base-inl.h

@@ -413,6 +413,7 @@ class Yield : public GenImpl<Value, Yield<Value, Source>> {
 template <class Value>
 class Empty : public GenImpl<Value, Empty<Value>> {
  public:
+  Empty() = default;
   template <class Handler>
   bool apply(Handler&&) const {
     return true;
@@ -1290,6 +1291,7 @@ class GroupByAdjacent : public Operator<GroupByAdjacent<Selector>> {
 template <class Expected>
 class TypeAssertion : public Operator<TypeAssertion<Expected>> {
  public:
+  TypeAssertion() = default;
   template <class Source, class Value>
   const Source& compose(const GenImpl<Value, Source>& source) const {
     static_assert(