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Use C++17 fold expression macro
This puts the fold expressions behind the feature macro instead of a general C++17 macro. It also adds a fold expression optimization to constexpr_sum (guarded by the same feature macro).
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@ -401,7 +401,7 @@ template <bool B> using bool_constant = std::integral_constant<bool, B>;
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template <typename T> struct negation : bool_constant<!T::value> { };
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/// Compile-time all/any/none of that check the boolean value of all template types
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#ifdef PYBIND11_CPP17
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#ifdef __cpp_fold_expressions
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template <class... Ts> using all_of = bool_constant<(Ts::value && ...)>;
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template <class... Ts> using any_of = bool_constant<(Ts::value || ...)>;
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#elif !defined(_MSC_VER)
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@ -444,9 +444,13 @@ struct void_type { };
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template <typename...> struct type_list { };
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/// Compile-time integer sum
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#ifdef __cpp_fold_expressions
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template <typename... Ts> constexpr size_t constexpr_sum(Ts... ns) { return (0 + ... + size_t{ns}); }
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#else
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constexpr size_t constexpr_sum() { return 0; }
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template <typename T, typename... Ts>
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constexpr size_t constexpr_sum(T n, Ts... ns) { return size_t{n} + constexpr_sum(ns...); }
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#endif
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NAMESPACE_BEGIN(constexpr_impl)
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/// Implementation details for constexpr functions
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