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+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+
+#include <tuple>
+#include <type_traits>
+
+namespace lsp {
+namespace traits {
+
+// NthTypeOf returns the `N`th type in `Types`
+template <int N, typename... Types>
+using NthTypeOf = typename std::tuple_element<N, std::tuple<Types...>>::type;
+
+// `IsTypeOrDerived<BASE, T>::value` is true iff `T` is of type `BASE`, or
+// derives from `BASE`.
+template <typename BASE, typename T>
+using IsTypeOrDerived = std::integral_constant<
+ bool,
+ std::is_base_of<BASE, typename std::decay<T>::type>::value ||
+ std::is_same<BASE, typename std::decay<T>::type>::value>;
+
+// `EachIsTypeOrDerived<N, BASES, TYPES>::value` is true iff all of the types in
+// the std::tuple `TYPES` is of, or derives from the corresponding indexed type
+// in the std::tuple `BASES`.
+// `N` must be equal to the number of types in both the std::tuple `BASES` and
+// `TYPES`.
+template <int N, typename BASES, typename TYPES>
+struct EachIsTypeOrDerived {
+ using base = typename std::tuple_element<N - 1, BASES>::type;
+ using type = typename std::tuple_element<N - 1, TYPES>::type;
+ using last_matches = IsTypeOrDerived<base, type>;
+ using others_match = EachIsTypeOrDerived<N - 1, BASES, TYPES>;
+ static constexpr bool value = last_matches::value && others_match::value;
+};
+
+// EachIsTypeOrDerived specialization for N = 1
+template <typename BASES, typename TYPES>
+struct EachIsTypeOrDerived<1, BASES, TYPES> {
+ using base = typename std::tuple_element<0, BASES>::type;
+ using type = typename std::tuple_element<0, TYPES>::type;
+ static constexpr bool value = IsTypeOrDerived<base, type>::value;
+};
+
+// EachIsTypeOrDerived specialization for N = 0
+template <typename BASES, typename TYPES>
+struct EachIsTypeOrDerived<0, BASES, TYPES> {
+ static constexpr bool value = true;
+};
+
+// Signature describes the signature of a function.
+template <typename RETURN, typename... PARAMETERS>
+struct Signature {
+ // The return type of the function signature
+ using ret = RETURN;
+ // The parameters of the function signature held in a std::tuple
+ using parameters = std::tuple<PARAMETERS...>;
+ // The type of the Nth parameter of function signature
+ template <std::size_t N>
+ using parameter = NthTypeOf<N, PARAMETERS...>;
+ // The total number of parameters
+ static constexpr std::size_t parameter_count = sizeof...(PARAMETERS);
+};
+
+// SignatureOf is a traits helper that infers the signature of the function,
+// method, static method, lambda, or function-like object `F`.
+template <typename F>
+struct SignatureOf {
+ // The signature of the function-like object `F`
+ using type = typename SignatureOf<decltype(&F::operator())>::type;
+};
+
+// SignatureOf specialization for a regular function or static method.
+template <typename R, typename... ARGS>
+struct SignatureOf<R (*)(ARGS...)> {
+ // The signature of the function-like object `F`
+ using type = Signature<typename std::decay<R>::type,
+ typename std::decay<ARGS>::type...>;
+};
+
+// SignatureOf specialization for a non-static method.
+template <typename R, typename C, typename... ARGS>
+struct SignatureOf<R (C::*)(ARGS...)> {
+ // The signature of the function-like object `F`
+ using type = Signature<typename std::decay<R>::type,
+ typename std::decay<ARGS>::type...>;
+};
+
+// SignatureOf specialization for a non-static, const method.
+template <typename R, typename C, typename... ARGS>
+struct SignatureOf<R (C::*)(ARGS...) const> {
+ // The signature of the function-like object `F`
+ using type = Signature<typename std::decay<R>::type,
+ typename std::decay<ARGS>::type...>;
+};
+
+// SignatureOfT is an alias to `typename SignatureOf<F>::type`.
+template <typename F>
+using SignatureOfT = typename SignatureOf<F>::type;
+
+// ParameterType is an alias to `typename SignatureOf<F>::type::parameter<N>`.
+template <typename F, std::size_t N>
+using ParameterType = typename SignatureOfT<F>::template parameter<N>;
+
+// `HasSignature<F, S>::value` is true iff the function-like `F` has a matching
+// signature to the function-like `S`.
+template <typename F, typename S>
+using HasSignature = std::integral_constant<
+ bool,
+ std::is_same<SignatureOfT<F>, SignatureOfT<S>>::value>;
+
+// `Min<A, B>::value` resolves to the smaller value of A and B.
+template <std::size_t A, std::size_t B>
+using Min = std::integral_constant<std::size_t, (A < B ? A : B)>;
+
+// `CompatibleWith<F, S>::value` is true iff the function-like `F`
+// can be called with the argument types of the function-like `S`. Return type
+// of the two functions are not considered.
+template <typename F, typename S>
+using CompatibleWith = std::integral_constant<
+ bool,
+ (SignatureOfT<S>::parameter_count == SignatureOfT<F>::parameter_count) &&
+ EachIsTypeOrDerived<Min<SignatureOfT<S>::parameter_count,
+ SignatureOfT<F>::parameter_count>::value,
+ typename SignatureOfT<S>::parameters,
+ typename SignatureOfT<F>::parameters>::value>;
+
+// If `CONDITION` is true then EnableIf resolves to type T, otherwise an
+// invalid type.
+template <bool CONDITION, typename T = void>
+using EnableIf = typename std::enable_if<CONDITION, T>::type;
+
+// If `BASE` is a base of `T` then EnableIfIsType resolves to type `TRUE`,
+// otherwise an invalid type.
+template <typename BASE, typename T, typename TRUE_ = void>
+using EnableIfIsType = EnableIf<IsTypeOrDerived<BASE, T>::value, TRUE_>;
+
+// If the function-like `F` has a matching signature to the function-like `S`
+// then EnableIfHasSignature resolves to type `TRUE`, otherwise an invalid type.
+template <typename F, typename S, typename TRUE_ = void>
+using EnableIfHasSignature = EnableIf<HasSignature<F, S>::value, TRUE_>;
+
+} // namespace traits
+} // namespace lsp
+