1 files changed, 116 insertions, 0 deletions
diff --git a/mfbt/FunctionTypeTraits.h b/mfbt/FunctionTypeTraits.h
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+++ b/mfbt/FunctionTypeTraits.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Helpers to manipulate function types that don't fit in TypeTraits.h */
+
+#ifndef mozilla_FunctionTypeTraits_h
+#define mozilla_FunctionTypeTraits_h
+
+#include <cstddef> /* for size_t */
+#include <tuple>
+
+namespace mozilla {
+
+// Main FunctionTypeTraits declaration, taking one template argument.
+//
+// Given a function type, FunctionTypeTraits will expose the following members:
+// - ReturnType: Return type.
+// - arity: Number of parameters (size_t).
+// - ParameterType<N>: Type of the Nth** parameter, 0-indexed.
+//
+// ** `ParameterType<N>` with `N` >= `arity` is allowed and gives `void`.
+// This prevents compilation errors when trying to access a type outside of the
+// function's parameters, which is useful for parameters checks, e.g.:
+//   template<typename F>
+//   auto foo(F&&)
+//    -> enable_if(FunctionTypeTraits<F>::arity == 1 &&
+//                 is_same<FunctionTypeTraits<F>::template ParameterType<0>,
+//                         int>::value,
+//                 void)
+//   {
+//     // This function will only be enabled if `F` takes one `int`.
+//     // Without the permissive ParameterType<any N>, it wouldn't even compile.
+//
+// Note: FunctionTypeTraits does not work with generic lambdas `[](auto&) {}`,
+// because parameter types cannot be known until an actual invocation when types
+// are inferred from the given arguments.
+template <typename T>
+struct FunctionTypeTraits;
+
+// Remove reference and pointer wrappers, if any.
+template <typename T>
+struct FunctionTypeTraits<T&> : public FunctionTypeTraits<T> {};
+template <typename T>
+struct FunctionTypeTraits<T&&> : public FunctionTypeTraits<T> {};
+template <typename T>
+struct FunctionTypeTraits<T*> : public FunctionTypeTraits<T> {};
+
+// Extract `operator()` function from callables (e.g. lambdas, std::function).
+template <typename T>
+struct FunctionTypeTraits
+    : public FunctionTypeTraits<decltype(&T::operator())> {};
+
+namespace detail {
+
+// If `safe`, retrieve the `N`th type from `As`, otherwise `void`.
+// See top description for reason.
+template <bool safe, size_t N, typename... As>
+struct TupleElementSafe;
+template <size_t N, typename... As>
+struct TupleElementSafe<true, N, As...> {
+  using Type = typename std::tuple_element<N, std::tuple<As...>>::type;
+};
+template <size_t N, typename... As>
+struct TupleElementSafe<false, N, As...> {
+  using Type = void;
+};
+
+template <typename R, typename... As>
+struct FunctionTypeTraitsHelper {
+  using ReturnType = R;
+  static constexpr size_t arity = sizeof...(As);
+  template <size_t N>
+  using ParameterType =
+      typename TupleElementSafe<(N < sizeof...(As)), N, As...>::Type;
+};
+
+}  // namespace detail
+
+// Specialization for free functions.
+template <typename R, typename... As>
+struct FunctionTypeTraits<R(As...)>
+    : detail::FunctionTypeTraitsHelper<R, As...> {};
+
+// Specialization for non-const member functions.
+template <typename C, typename R, typename... As>
+struct FunctionTypeTraits<R (C::*)(As...)>
+    : detail::FunctionTypeTraitsHelper<R, As...> {};
+
+// Specialization for const member functions.
+template <typename C, typename R, typename... As>
+struct FunctionTypeTraits<R (C::*)(As...) const>
+    : detail::FunctionTypeTraitsHelper<R, As...> {};
+
+#ifdef NS_HAVE_STDCALL
+// Specialization for __stdcall free functions.
+template <typename R, typename... As>
+struct FunctionTypeTraits<R NS_STDCALL(As...)>
+    : detail::FunctionTypeTraitsHelper<R, As...> {};
+
+// Specialization for __stdcall non-const member functions.
+template <typename C, typename R, typename... As>
+struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...)>
+    : detail::FunctionTypeTraitsHelper<R, As...> {};
+
+// Specialization for __stdcall const member functions.
+template <typename C, typename R, typename... As>
+struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...) const>
+    : detail::FunctionTypeTraitsHelper<R, As...> {};
+#endif  // NS_HAVE_STDCALL
+
+}  // namespace mozilla
+
+#endif  // mozilla_FunctionTypeTraits_h