[libc-commits] [libc] [llvm] [libc] Rename cpp::function to cpp::function_ref (PR #236108)

[Jeff Bailey via libc-commits]

================
@@ -1,53 +1,64 @@
-//===-- Self contained functional header ------------------------*- 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
 //
 //===----------------------------------------------------------------------===//
+///
+/// \file
+/// Implementation of cpp::function_ref.
+///
+//===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC___SUPPORT_CPP_FUNCTIONAL_H
 #define LLVM_LIBC_SRC___SUPPORT_CPP_FUNCTIONAL_H
 
 #include "hdr/stdint_proxy.h"
 #include "src/__support/CPP/type_traits/enable_if.h"
 #include "src/__support/CPP/type_traits/is_convertible.h"
+#include "src/__support/CPP/type_traits/is_function.h"
+#include "src/__support/CPP/type_traits/is_pointer.h"
 #include "src/__support/CPP/type_traits/is_same.h"
 #include "src/__support/CPP/type_traits/is_void.h"
 #include "src/__support/CPP/type_traits/remove_cvref.h"
 #include "src/__support/CPP/type_traits/remove_reference.h"
+#include "src/__support/CPP/utility/declval.h"
 #include "src/__support/CPP/utility/forward.h"
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 namespace cpp {
 
-/// A function type adapted from LLVM's function_ref.
+/// A non-owning reference to a callable, similar to C++26 std::function_ref.
 /// This class does not own the callable, so it is not in general safe to
-/// store a function.
-template <typename Fn> class function;
+/// store a function_ref.
+template <typename Fn> class function_ref;
 
-template <typename Ret, typename... Params> class function<Ret(Params...)> {
-  Ret (*callback)(intptr_t callable, Params... params) = nullptr;
+template <typename Ret, typename... Params> class function_ref<Ret(Params...)> {
+  Ret (*callback)(intptr_t callable, Params... params);
   intptr_t callable;
 
   template <typename Callable>
   LIBC_INLINE static Ret callback_fn(intptr_t callable, Params... params) {
-    return (*reinterpret_cast<Callable *>(callable))(
-        cpp::forward<Params>(params)...);
+    // Discard the result if Ret is void, and convert it implicitly otherwise.
+    // Avoiding an explicit cast lets the compiler diagnose narrowing
+    // conversions and discarded [[nodiscard]] results.
+    if constexpr (cpp::is_void_v<Ret>)
+      (*reinterpret_cast<Callable *>(callable))(
----------------
kaladron wrote:

(optional nit) Both the if and else have to do this cast.  This could be pulled up to a temporary to make those a bit easier to read.

https://github.com/llvm/llvm-project/pull/236108