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Interconverting std::function with copyable_function

C++11 introduced std::function as a type-erased callable-object holder. function was badly designed in a few ways: Its rarely used “go fish” API bloats every user; it advertises operator() const even when the controlled object’s operator() is mutating; it handles what should be a precondition violation by throwing an exception, which again bloats every user and means its operator() can never be noexcept. So C++26 fixed these problems by adding std::copyable_function. (Which preserves some of function’s suboptimal design decisions: copyable_function<bool()> remains implicitly convertible to copyable_function<void()>, contextually convertible to bool, and assignable from nullptr.)

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