/*=============================================================================
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Copyright (c) 2014 Paul Fultz II
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reveal.h
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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==============================================================================*/
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#ifndef BOOST_HOF_GUARD_FUNCTION_REVEAL_H
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#define BOOST_HOF_GUARD_FUNCTION_REVEAL_H
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/// reveal
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/// ======
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///
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/// Description
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/// -----------
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///
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/// The `reveal` function adaptor helps shows the error messages that get
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/// masked on some compilers. Sometimes an error in a function that causes a
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/// substitution failure, will remove the function from valid overloads. On
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/// compilers without a backtrace for substitution failure, this will mask the
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/// error inside the function. The `reveal` adaptor will expose these error
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/// messages while still keeping the function SFINAE-friendly.
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///
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/// Sample
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/// ------
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///
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/// If we take the `print` example from the quick start guide like this:
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///
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/// namespace adl {
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///
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/// using std::begin;
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///
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/// template<class R>
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/// auto adl_begin(R&& r) BOOST_HOF_RETURNS(begin(r));
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/// }
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///
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/// BOOST_HOF_STATIC_LAMBDA_FUNCTION(for_each_tuple) = [](const auto& sequence, auto f) BOOST_HOF_RETURNS
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/// (
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/// boost::hof::unpack(boost::hof::proj(f))(sequence)
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/// );
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///
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/// auto print = boost::hof::fix(boost::hof::first_of(
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/// [](auto, const auto& x) -> decltype(std::cout << x, void())
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/// {
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/// std::cout << x << std::endl;
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/// },
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/// [](auto self, const auto& range) -> decltype(self(*adl::adl_begin(range)), void())
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/// {
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/// for(const auto& x:range) self(x);
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/// },
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/// [](auto self, const auto& tuple) -> decltype(for_each_tuple(tuple, self), void())
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/// {
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/// return for_each_tuple(tuple, self);
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/// }
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/// ));
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///
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/// Which prints numbers and vectors:
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///
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/// print(5);
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///
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/// std::vector<int> v = { 1, 2, 3, 4 };
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/// print(v);
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///
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/// However, if we pass a type that can't be printed, we get an error like
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/// this:
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///
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/// print.cpp:49:5: error: no matching function for call to object of type 'boost::hof::fix_adaptor<boost::hof::first_of_adaptor<(lambda at print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)> >'
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/// print(foo{});
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/// ^~~~~
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/// fix.hpp:158:5: note: candidate template ignored: substitution failure [with Ts = <foo>]: no matching function for call to object of type 'const boost::hof::first_of_adaptor<(lambda at
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/// print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)>'
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/// operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
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///
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/// Which is short and gives very little information why it can't be called.
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/// It doesn't even show the overloads that were try. However, using the
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/// `reveal` adaptor we can get more info about the error like this:
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///
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/// print.cpp:49:5: error: no matching function for call to object of type 'boost::hof::reveal_adaptor<boost::hof::fix_adaptor<boost::hof::first_of_adaptor<(lambda at print.cpp:29:9), (lambda at print.cpp:33:9),
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/// (lambda at print.cpp:37:9)> >, boost::hof::fix_adaptor<boost::hof::first_of_adaptor<(lambda at print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)> > >'
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/// boost::hof::reveal(print)(foo{});
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/// ^~~~~~~~~~~~~~~~~~
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/// reveal.hpp:149:20: note: candidate template ignored: substitution failure [with Ts = <foo>, $1 = void]: no matching function for call to object of type '(lambda at print.cpp:29:9)'
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/// constexpr auto operator()(Ts&&... xs) const
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/// ^
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/// reveal.hpp:149:20: note: candidate template ignored: substitution failure [with Ts = <foo>, $1 = void]: no matching function for call to object of type '(lambda at print.cpp:33:9)'
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/// constexpr auto operator()(Ts&&... xs) const
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/// ^
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/// reveal.hpp:149:20: note: candidate template ignored: substitution failure [with Ts = <foo>, $1 = void]: no matching function for call to object of type '(lambda at print.cpp:37:9)'
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/// constexpr auto operator()(Ts&&... xs) const
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/// ^
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/// fix.hpp:158:5: note: candidate template ignored: substitution failure [with Ts = <foo>]: no matching function for call to object of type 'const boost::hof::first_of_adaptor<(lambda at
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/// print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)>'
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/// operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
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///
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/// So now the error has a note for each of the lambda overloads it tried. Of
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/// course this can be improved even further by providing custom reporting of
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/// failures.
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///
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/// Synopsis
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/// --------
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///
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/// template<class F>
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/// reveal_adaptor<F> reveal(F f);
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///
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/// Requirements
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/// ------------
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///
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/// F must be:
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///
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/// * [ConstInvocable](ConstInvocable)
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/// * MoveConstructible
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///
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/// Reporting Failures
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/// ------------------
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///
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/// By default, `reveal` reports the substitution failure by trying to call
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/// the function. However, more detail expressions can be be reported from a
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/// template alias by using `as_failure`. This is done by defining a nested
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/// `failure` struct in the function object and then inheriting from
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/// `as_failure`. Also multiple failures can be reported by using
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/// `with_failures`.
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///
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/// Synopsis
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/// --------
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///
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/// // Report failure by instantiating the Template
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/// template<template<class...> class Template>
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/// struct as_failure;
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///
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/// // Report multiple falures
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/// template<class... Failures>
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/// struct with_failures;
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///
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/// // Report the failure for each function
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/// template<class... Fs>
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/// struct failure_for;
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///
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/// // Get the failure of a function
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/// template<class F>
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/// struct get_failure;
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///
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/// Example
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/// -------
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///
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/// #include <boost/hof.hpp>
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/// #include <cassert>
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///
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/// struct sum_f
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/// {
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/// template<class T, class U>
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/// using sum_failure = decltype(std::declval<T>()+std::declval<U>());
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///
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/// struct failure
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/// : boost::hof::as_failure<sum_failure>
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/// {};
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///
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/// template<class T, class U>
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/// auto operator()(T x, U y) const BOOST_HOF_RETURNS(x+y);
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/// };
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///
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/// int main() {
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/// assert(sum_f()(1, 2) == 3);
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/// }
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///
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#include <boost/hof/always.hpp>
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#include <boost/hof/returns.hpp>
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#include <boost/hof/is_invocable.hpp>
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#include <boost/hof/identity.hpp>
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#include <boost/hof/detail/move.hpp>
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#include <boost/hof/detail/callable_base.hpp>
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#include <boost/hof/detail/delegate.hpp>
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#include <boost/hof/detail/holder.hpp>
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#include <boost/hof/detail/join.hpp>
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#include <boost/hof/detail/make.hpp>
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#include <boost/hof/detail/static_const_var.hpp>
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#include <boost/hof/detail/using.hpp>
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#ifndef BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
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#ifdef __clang__
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#define BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS 1
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#else
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#define BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS 0
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#endif
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#endif
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namespace boost { namespace hof {
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namespace detail {
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template<class T, class=void>
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struct has_failure
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: std::false_type
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{};
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template<class T>
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struct has_failure<T, typename holder<
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typename T::failure
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>::type>
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: std::true_type
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{};
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struct identity_failure
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{
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template<class T>
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T operator()(T&& x);
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template<class T>
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static T&& val();
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#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
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template<template<class...> class Template, class... Ts>
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BOOST_HOF_USING(defer, Template<Ts...>);
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#else
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template<template<class...> class Template, class... Ts>
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static auto defer(Ts&&...) -> Template<Ts...>;
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#endif
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};
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}
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template<class F, class=void>
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struct get_failure
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{
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template<class... Ts>
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struct of
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{
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#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
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template<class Id>
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using apply = decltype(Id()(std::declval<F>())(std::declval<Ts>()...));
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#else
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template<class Id>
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static auto apply(Id id) -> decltype(id(std::declval<F>())(std::declval<Ts>()...));
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#endif
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};
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};
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template<class F>
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struct get_failure<F, typename std::enable_if<detail::has_failure<F>::value>::type>
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: F::failure
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{};
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template<template<class...> class Template>
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struct as_failure
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{
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template<class... Ts>
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struct of
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{
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#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
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template<class Id>
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using apply = typename Id::template defer<Template, Ts...>;
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#else
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template<class Id>
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static auto apply(Id) -> decltype(Id::template defer<Template, Ts...>());
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#endif
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};
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};
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namespace detail {
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template<class Failure, class... Ts>
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BOOST_HOF_USING_TYPENAME(apply_failure, Failure::template of<Ts...>);
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template<class F, class Failure>
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struct reveal_failure
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{
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// Add default constructor to make clang 3.4 happy
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constexpr reveal_failure()
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{}
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// This is just a placeholder to produce a note in the compiler, it is
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// never called
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template<
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class... Ts,
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class=typename std::enable_if<(!is_invocable<F, Ts...>::value)>::type
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>
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constexpr auto operator()(Ts&&... xs) const
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#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
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-> typename apply_failure<Failure, Ts...>::template apply<boost::hof::detail::identity_failure>;
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#else
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-> decltype(apply_failure<Failure, Ts...>::apply(boost::hof::detail::identity_failure()));
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#endif
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};
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template<class F, class Failure=get_failure<F>, class=void>
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struct traverse_failure
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: reveal_failure<F, Failure>
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{
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constexpr traverse_failure()
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{}
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};
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template<class F, class Failure>
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struct traverse_failure<F, Failure, typename holder<
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typename Failure::children
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>::type>
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: Failure::children::template overloads<F>
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{
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constexpr traverse_failure()
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{}
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};
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template<class Failure, class Transform, class=void>
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struct transform_failures
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: Transform::template apply<Failure>
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{};
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template<class Failure, class Transform>
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struct transform_failures<Failure, Transform, typename holder<
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typename Failure::children
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>::type>
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: Failure::children::template transform<Transform>
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{};
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}
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template<class Failure, class... Failures>
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struct failures;
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template<class... Fs>
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struct with_failures
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{
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typedef BOOST_HOF_JOIN(failures, Fs...) children;
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};
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template<class Failure, class... Failures>
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struct failures
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{
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template<class Transform>
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BOOST_HOF_USING(transform, with_failures<detail::transform_failures<Failure, Transform>, detail::transform_failures<Failures, Transform>...>);
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template<class F, class FailureBase=BOOST_HOF_JOIN(failures, Failures...)>
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struct overloads
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: detail::traverse_failure<F, Failure>, FailureBase::template overloads<F>
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{
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constexpr overloads()
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{}
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using detail::traverse_failure<F, Failure>::operator();
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using FailureBase::template overloads<F>::operator();
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};
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};
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template<class Failure>
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struct failures<Failure>
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{
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template<class Transform>
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BOOST_HOF_USING(transform, with_failures<detail::transform_failures<Failure, Transform>>);
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template<class F>
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BOOST_HOF_USING(overloads, detail::traverse_failure<F, Failure>);
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};
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template<class Transform, class... Fs>
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struct failure_map
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: with_failures<detail::transform_failures<get_failure<Fs>, Transform>...>
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{};
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template<class... Fs>
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struct failure_for
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: with_failures<get_failure<Fs>...>
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{};
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template<class F, class Base=detail::callable_base<F>>
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struct reveal_adaptor
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: detail::traverse_failure<Base>, Base
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{
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typedef reveal_adaptor fit_rewritable1_tag;
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using detail::traverse_failure<Base>::operator();
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using Base::operator();
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BOOST_HOF_INHERIT_CONSTRUCTOR(reveal_adaptor, Base);
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};
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// Avoid double reveals, it causes problem on gcc 4.6
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template<class F>
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struct reveal_adaptor<reveal_adaptor<F>>
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: reveal_adaptor<F>
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{
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typedef reveal_adaptor fit_rewritable1_tag;
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BOOST_HOF_INHERIT_CONSTRUCTOR(reveal_adaptor, reveal_adaptor<F>);
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};
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BOOST_HOF_DECLARE_STATIC_VAR(reveal, detail::make<reveal_adaptor>);
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}} // namespace boost::hof
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#endif
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