The concept for operator= is too loose.

[poor-circle]

I trying to use tl::expected<tuplet::tuple<...>,...>'s operator= and it failed.

The expected use SFINAE(std::is_assignable<T,U>) in operator= to determine if is unexpected error or expected value. However, the concept for tuplet::tuple is too loose so that it accept every other than type, which cause compile error.

Tightening the concept will fix it.