[llvm] [libc] Add invoke / invoke_result type traits (PR #65750)

[Clement Courbet via llvm-commits]

================
@@ -0,0 +1,58 @@
+//===-- invoke type_traits --------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_CPP_TYPE_TRAITS_INVOKE_H
+#define LLVM_LIBC_SRC_SUPPORT_CPP_TYPE_TRAITS_INVOKE_H
+
+#include "src/__support/CPP/type_traits/decay.h"
+#include "src/__support/CPP/type_traits/is_base_of.h"
+#include "src/__support/CPP/utility/forward.h"
+
+// BEWARE : this implementation is not fully conformant as it doesn't take
+// `cpp::reference_wrapper` into account.
+
+namespace __llvm_libc::cpp {
+
+namespace detail {
+
+// Catch all function types.
+template <class FunctionPtrType> struct invoke_dispatcher {
+  template <class... Args>
+  static auto call(FunctionPtrType &&fun, Args &&...args) {
+    return cpp::forward<FunctionPtrType>(fun)(cpp::forward<Args>(args)...);
+  }
+};
+
+// Catch pointer to member function types.
+template <class Class, class FunctionReturnType>
+struct invoke_dispatcher<FunctionReturnType Class::*> {
+  using FunctionPtrType = FunctionReturnType Class::*;
+
+  template <class T, class... Args, class DecayT = cpp::decay_t<T>>
+  static auto call(FunctionPtrType fun, T &&t1, Args &&...args) {
+    if constexpr (cpp::is_base_of_v<Class, DecayT>) {
+      // T is a (possibly cv ref) type.
+      return (cpp::forward<T>(t1).*fun)(cpp::forward<Args>(args)...);
+    } else {
+      // T is assumed to be a pointer type.
+      return (*cpp::forward<T>(t1).*fun)(cpp::forward<Args>(args)...);
+    }
+  }
+};
+
+} // namespace detail
+
+template <class Function, class... Args>
+auto invoke(Function &&fun, Args &&...args) {
+  return detail::invoke_dispatcher<cpp::decay_t<Function>>::call(
+      cpp::forward<Function>(fun), cpp::forward<Args>(args)...);
----------------
legrosbuffle wrote:

because `call` takes an rvalue reference, if `fun` is deduced as a `const&`, this will probably not compile (some thing like "cannot bind const& to &&")

https://github.com/llvm/llvm-project/pull/65750