LWG4345 task::promise_type::return_value default template parameter

Fixes NB US 251-388 (C++26 CD).

Author: eisenwave

source/exec.tex

@@ -7430,7 +7430,7 @@
     coroutine_handle<> unhandled_stopped() noexcept;
 
     void return_void();                 // present only if \tcode{is_void_v<T>} is \tcode{true}
-    template<class V>
+    template<class V = T>
       void return_value(V&& value);     // present only if \tcode{is_void_v<T>} is \tcode{false}
 
     template<class E>