Fix implementation of optional<T&>::emplace()

include/tl/optional.hpp

@@ -1982,15 +1982,12 @@ public:
     return *this;
   }
 
-  /// Constructs the value in-place, destroying the current one if there is
-  /// one.
-  template <class... Args> T &emplace(Args &&... args) noexcept {
-    static_assert(std::is_constructible<T, Args &&...>::value,
-                  "T must be constructible with Args");
-
-    *this = nullopt;
-    this->construct(std::forward<Args>(args)...);
-    return value();
+  /// Rebinds this optional to `u`.
+  template <class U = T,
+            detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>
+                * = nullptr>
+  optional &emplace(U &&u) noexcept {
+    return *this = std::forward<U>(u);
   }
 
   void swap(optional &rhs) noexcept { std::swap(m_value, rhs.m_value); }