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+/* Copyright 2017 Joaquin M Lopez Munoz.
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * http://www.boost.org/LICENSE_1_0.txt)
+ *
+ * See http://www.boost.org/libs/poly_collection for library home page.
+ */
+
+#ifndef BOOST_POLY_COLLECTION_DETAIL_IS_LIKELY_STATELESS_LAMBDA_HPP
+#define BOOST_POLY_COLLECTION_DETAIL_IS_LIKELY_STATELESS_LAMBDA_HPP
+
+#if defined(_MSC_VER)
+#pragma once
+#endif
+
+#include <type_traits>
+
+namespace boost{
+
+namespace poly_collection{
+
+namespace detail{
+
+/* Stateless lambda expressions have one (and only one) call operator and are
+ * convertible to a function pointer with the same signature. Non-lambda types
+ * could satisfy this too, hence the "likely" qualifier.
+ */
+
+template<typename T>
+struct has_one_operator_call_helper
+{
+ template<typename Q> static std::true_type test(decltype(&Q::operator())*);
+ template<typename> static std::false_type test(...);
+
+ using type=decltype(test<T>(nullptr));
+};
+
+template<typename T>
+using has_one_operator_call=typename has_one_operator_call_helper<T>::type;
+
+template<typename T>
+struct equivalent_function_pointer
+{
+ template<typename Q,typename R,typename... Args>
+ static auto helper(R (Q::*)(Args...)const)->R(*)(Args...);
+ template<typename Q,typename R,typename... Args>
+ static auto helper(R (Q::*)(Args...))->R(*)(Args...);
+
+ using type=decltype(helper(&T::operator()));
+};
+
+template<typename T,typename=void>
+struct is_likely_stateless_lambda:std::false_type{};
+
+template<typename T>
+struct is_likely_stateless_lambda<
+ T,
+ typename std::enable_if<has_one_operator_call<T>::value>::type
+>:std::is_convertible<
+ T,
+ typename equivalent_function_pointer<T>::type
+>{};
+
+} /* namespace poly_collection::detail */
+
+} /* namespace poly_collection */
+
+} /* namespace boost */
+
+#endif