Generic detection of empty-callable in Function ctor
Summary: [Folly] Generic detection of empty-callable in `Function` ctor. Constructing a `folly::Function` from an empty `std::function` should result in an empty object. However, it results in a full object which, when invoked, throws `std::bad_function_call`. This may be a problem in some cases which need to use the emptiness/fullness property to tell whether `std::bad_function_call` would be thrown if the object were to be invoked. This solution proposes a new protocol: to check arguments of all types, not just pointers, for constructibility-from and equality-comparability-with `nullptr`, and then if those two checks pass, to check equality-comparison-with `nullptr`. If the argument type is is constructible from `nullptr` and is equality-comparable with `nullptr` and compares equal to `nullptr, then treat the argument as empty, i.e., as if it were `nullptr`. This way, an empty `std::function` gets treated as if it were `nullptr` - as well as any other custom function object type out there - without having to enumerate every one of them. The new protocol is somewhat strict. An alternative to the new protocol is to check if the object is castable to `bool` and, if it is, cast to `bool`, but such a protocol is broader than the one proposed in this diff. Fixes #886. Reviewed By: nbronson Differential Revision: D9287898 fbshipit-source-id: bcb574387122aac92d154e81732e82ddbcdd4915
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