[libc++] Make memory_resource allocate [[nodiscard]]

[J-Camilleri]

Summary

P0600: Implement the bit for [mem.res.class]

[philnik]

This isn't implementing P0600. P0600 is against N4700, which was the C++ working draft at some point. experimental/memory_resource implements LFTSv2, where memory_resource::allocate isn't [[nodiscard]].

[J-Camilleri]

In D131952#3725591, @philnik wrote:

This isn't implementing P0600. P0600 is against N4700, which was the C++ working draft at some point. experimental/memory_resource implements LFTSv2, where memory_resource::allocate isn't [[nodiscard]].

Thanks for the quick reply and clarification. It is my first time trying to contribute here and I did not know the about the distinction.

Am I correct in thinking that selecting 'Abandon Revision' as an action is enough to decline this?

[philnik]

In D131952#3726190, @J-Camilleri wrote:

In D131952#3725591, @philnik wrote:

This isn't implementing P0600. P0600 is against N4700, which was the C++ working draft at some point. experimental/memory_resource implements LFTSv2, where memory_resource::allocate isn't [[nodiscard]].

Thanks for the quick reply and clarification. It is my first time trying to contribute here and I did not know the about the distinction.

No worries. The difference is indeed quite subtle.

Am I correct in thinking that selecting 'Abandon Revision' as an action is enough to decline this?

Yes, that's enough.

libcxx/docs/Status/Cxx20.rst

 
 .. csv-table::
    :file: Cxx20Papers.csv
    :header-rows: 1
    :widths: auto
 
 .. note::
 
-   .. [#note-P0600] P0600: The missing bits in P0600 are in |sect|\ [mem.res.class] and |sect|\ [mem.poly.allocator.class].
+   .. [#note-P0600] P0600: The missing bit in P0600 is |sect|\ [mem.poly.allocator.class].
    .. [#note-P0645] P0645: The paper is implemented but still marked as an incomplete feature
       (the feature-test macro is not set and the libary is only available when built with ``-fexperimental-library``).
       Not yet implemented LWG-issues will cause API and ABI breakage.
    .. [#note-P0966] P0966: It was previously erroneously marked as complete in version 8.0. See `bug 45368 <https://llvm.org/PR45368>`__.
    .. [#note-P0619] P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, D.12, and D.14 remain undone.
    .. [#note-P0883.1] P0883: shared_ptr and floating-point changes weren't applied as they themselves aren't implemented yet.
    .. [#note-P0883.2] P0883: ``ATOMIC_FLAG_INIT`` was marked deprecated in version 14.0, but was undeprecated with the implementation of LWG3659 in version 15.0.
    .. [#note-P2231] P2231: Optional is complete. The changes to variant haven't been implemented yet.

libcxx/include/experimental/memory_resource

 class _LIBCPP_TYPE_VIS memory_resource
 {
     static const size_t __max_align = _LIBCPP_ALIGNOF(max_align_t);
 
 // 8.5.2, memory.resource.public
 public:
     virtual ~memory_resource() = default;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
     void* allocate(size_t __bytes, size_t __align = __max_align)
         { return do_allocate(__bytes, __align); }
 
     _LIBCPP_INLINE_VISIBILITY
     void deallocate(void * __p, size_t __bytes, size_t __align = __max_align)
         { do_deallocate(__p, __bytes, __align); }
 
     _LIBCPP_INLINE_VISIBILITY

libcxx/test/libcxx/experimental/memory/memory.resource.class/allocate.cxx2a.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <experimental/memory_resource>
+
+// memory_resource:
+// void* allocate(size_t __bytes, size_t __align = __max_align)
+
+#include <experimental/memory_resource>
+
+#include "test_memory_resource.h"
+
+int main(int, char**) {
+  NullResource R;
+  R.allocate(3); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  return 0;
+}