https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77288
--- Comment #4 from dawid_jurek at vp dot pl --- After applying my patch code snippet you provided compile, run and works as expected. To be more precise I'm talking about such snippet: std::experimental::optional<std::string> os; os = "meow"; assert(bool(os) && *os == "meow"); You are right that operator=(_Up&& __u) won't be choose here as overload candidate. Anyway there is converting constructor: template <typename _Up, .. > constexpr optional(_Up&& __t) and generated move constructor: optional<_Tp>::optional(optional<_Tp>::optional<_Tp>&& ) Let's see what happen when flow achieve expression: os = "meow" 1. Call optional::optional<_Tp>::optional(_Up&& __t) where _Tp = std::string, _Up = char const (&) [13] for r-value "meow" and create temporary optional<std::string> containing "meow". 2. Call generated optional::optional<_Tp>::optional(optional<_Tp>::optional<_Tp>&& ) where _Tp = std::string and just transfer ownership from temporary optional to os. 3. Now os is engaged, os contains "meow" and assertion is happy. Regards, Dawid
