Fix null function check for `move_only_function` (#3038)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/functional

@@ -1383,20 +1383,19 @@ public:
         // (The RTTI savings may be significant as with lambdas and binds there may be many distinct callable types.
         // Here we don't have a distinct wrapper class for each callable type, only distinct functions when needed.)
 
+        // _Move and _Destroy are nullptr if trivial. Besides being an optimization, this enables assigning an
+        // empty function from a DLL that is unloaded later, and then safely moving/destroying that empty function.
+
         // Calls target
         typename _Invoke_t<_Noexcept>::_Call _Invoke;
-        // Moves the data, including pointer to "vtable", AND destroys old data (not resetting its "vtable")
+        // Moves the data, including pointer to "vtable", AND destroys old data (not resetting its "vtable").
+        // nullptr if we can trivially move two pointers.
         void(__stdcall* _Move)(_Move_only_function_data&, _Move_only_function_data&) _NOEXCEPT_FNPTR;
-        // Destroys data (not resetting its "vtable")
+        // Destroys data (not resetting its "vtable").
+        // nullptr if destruction is a no-op.
         void(__stdcall* _Destroy)(_Move_only_function_data&) _NOEXCEPT_FNPTR;
     };
 
-    static constexpr _Impl_t _Null_move_only_function = {
-        _Function_not_callable<_Rx, _Types...>,
-        nullptr,
-        nullptr,
-    };
-
     _Move_only_function_data _Data;
 
     _Move_only_function_base() noexcept = default; // leaves fields uninitialized
@@ -1407,12 +1406,12 @@ public:
     }
 
     void _Construct_with_null() noexcept {
-        _Data._Impl = &_Null_move_only_function;
+        _Data._Impl = nullptr;
         _Data._Set_large_fn_ptr(nullptr); // initialize, since we'll be copying it
     }
 
     void _Reset_to_null() noexcept {
-        _Data._Impl = &_Null_move_only_function;
+        _Data._Impl = nullptr;
     }
 
     template <class _Vt, class _VtInvQuals, class... _CTypes>
@@ -1426,14 +1425,14 @@ public:
     }
 
     static void _Checked_destroy(_Move_only_function_data& _Data) noexcept {
-        const auto _Impl = static_cast<const _Impl_t*>(_Data._Impl);
+        const auto _Impl = _Get_impl(_Data);
         if (_Impl->_Destroy) {
             _Impl->_Destroy(_Data);
         }
     }
 
     static void _Checked_move(_Move_only_function_data& _Data, _Move_only_function_data& _Src) noexcept {
-        const auto _Impl = static_cast<const _Impl_t*>(_Src._Impl);
+        const auto _Impl = _Get_impl(_Src);
         if (_Impl->_Move) {
             _Impl->_Move(_Data, _Src);
         } else {
@@ -1447,8 +1446,9 @@ public:
         // It is more efficient to do the reverse - this way no temporary storage for the old target will be used.
         // In some cases when some operations are trivial, it can be optimized,
         // as the order change is unobservable, and everything is noexcept here.
-        const auto _Other_impl_move   = static_cast<const _Impl_t*>(_Other._Data._Impl)->_Move;
+        const auto _This_impl         = _Get_impl(_Data);
-        const auto _This_impl_destroy = static_cast<const _Impl_t*>(_Data._Impl)->_Destroy;
+        const auto _Other_impl_move   = _Get_impl(_Other._Data)->_Move;
+        const auto _This_impl_destroy = _This_impl->_Destroy;
 
         if (!_Other_impl_move) {
             // Move is trivial, destroy first if needed
@@ -1462,7 +1462,13 @@ public:
         } else {
             // General case involving a temporary
             _Move_only_function_data _Tmp;
-            _Checked_move(_Tmp, _Data);
+
+            if (_This_impl->_Move) {
+                _This_impl->_Move(_Tmp, _Data);
+            } else {
+                _Function_move_large(_Tmp, _Data);
+            }
+
             _Other_impl_move(_Data, _Other._Data);
             _This_impl_destroy(_Tmp);
         }
@@ -1478,7 +1484,7 @@ public:
     }
 
     _NODISCARD bool _Is_null() const noexcept {
-        return _Data._Impl == &_Null_move_only_function;
+        return _Data._Impl == nullptr;
     }
 
     template <class _Vt>
@@ -1487,7 +1493,18 @@ public:
         || sizeof(_Vt) > _Move_only_function_data::_Buf_size<_Vt> || !is_nothrow_move_constructible_v<_Vt>;
 
     _NODISCARD auto _Get_invoke() const noexcept {
-        return static_cast<const _Impl_t*>(_Data._Impl)->_Invoke;
+        return _Get_impl(_Data)->_Invoke;
+    }
+
+    _NODISCARD static const _Impl_t* _Get_impl(const _Move_only_function_data& _Data) noexcept {
+        static constexpr _Impl_t _Null_move_only_function = {
+            _Function_not_callable<_Rx, _Types...>,
+            nullptr,
+            nullptr,
+        };
+
+        const auto _Ret = static_cast<const _Impl_t*>(_Data._Impl);
+        return _Ret ? _Ret : &_Null_move_only_function;
     }
 
     template <class _Vt, class _VtInvQuals>