[ PATCH ] [ C++ ] Implementing P0767 - deprecate POD

[Jonathan Wakely]

On 04/12/19 17:56 -0500, JeanHeyd Meneide wrote:
>Dear Jonathan,
>
>    Done! re-patched, below.

Thanks!

Some comments below.

>Best,
>JeanHeyd
>
>2019-12-03  JeanHeyd "ThePhD" Meneide  <phdofthehouse@gmail.com>
>
>        * include/bits/c++config: Add new _GLIBCXX20_DEPRECATED macro.
>        * include/std/type_traits: Deprecate is_pod with message.
>        * testuite/20_util/is_pod/deprecated-2a.cc (new): test deprecation

The parenthesis after the filename should tell you the entity that was
changed, so something like:

	* include/bits/c++config (_GLIBCXX20_DEPRECATED): Add new macro.
	* include/std/type_traits (is_pod, is_pod_v): Deprecate for C++20.
	* testuite/20_util/is_pod/deprecated-2a.cc: New test.

This tells you that _GLIBCXX20_DEPRECATED is the new macro, and that
is_pod and is_pod_v were deprecated.

>diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
>index 7ccfc5f199d..9e85fc2c9e8 100644
>--- a/libstdc++-v3/include/bits/c++config
>+++ b/libstdc++-v3/include/bits/c++config
>@@ -78,6 +78,7 @@
> //   _GLIBCXX_USE_DEPRECATED
> //   _GLIBCXX_DEPRECATED
> //   _GLIBCXX17_DEPRECATED
>+//   _GLIBCXX20_DEPRECATED( STRINGS... )
> #ifndef _GLIBCXX_USE_DEPRECATED
> # define _GLIBCXX_USE_DEPRECATED 1
> #endif
>@@ -94,6 +95,12 @@
> # define _GLIBCXX17_DEPRECATED
> #endif
>
>+#if defined(__DEPRECATED) && (__cplusplus > 201703L)
>+# define _GLIBCXX20_DEPRECATED(MSG) [[deprecated(MSG)]]
>+#else
>+# define _GLIBCXX20_DEPRECATED(MSG)
>+#endif
>+
> // Macros for ABI tag attributes.
> #ifndef _GLIBCXX_ABI_TAG_CXX11
> # define _GLIBCXX_ABI_TAG_CXX11 __attribute ((__abi_tag__ ("cxx11")))
>diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
>index 8e787a994c3..5980b21c763 100644
>--- a/libstdc++-v3/include/std/type_traits
>+++ b/libstdc++-v3/include/std/type_traits
>@@ -685,10 +685,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	"template argument must be a complete class or an unbounded array");
>     };
>
>-  /// is_pod
>+  /// is_pod (deprecated C++2a)
>   // Could use is_standard_layout && is_trivial instead of the builtin.
>   template<typename _Tp>
>-    struct is_pod
>+    struct
>+    _GLIBCXX20_DEPRECATED("is_pod<T> is deprecated in C++20: use (is_standard_layout_v<T> && is_trivial_v<T>) instead")
>+    is_pod
>     : public integral_constant<bool, __is_pod(_Tp)>
>     {
>       static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>@@ -3073,6 +3075,7 @@ template <typename _Tp>
> template <typename _Tp>
>   inline constexpr bool is_standard_layout_v = is_standard_layout<_Tp>::value;
> template <typename _Tp>
>+  _GLIBCXX20_DEPRECATED("is_pod_v<T> is deprecated in C++20: use (is_standard_layout_v<T> && is_trivial_v<T>) instead")

This message ends up repeating the "is deprecated":

pod.cc:6:21: warning: 'std::is_pod_v<A>' is deprecated: is_pod_v<T> is deprecated in C++20: use (is_standard_layout_v<T> && is_trivial_v<T>) instead [-Wdeprecated-declarations]

I think we can just shorten it to the "use ... instead". Also, given
that the compiler prints the template argument list "<A>" it's a bit
confusing to also put argument lists (with different types) in the
message. I think this is better:

pod.cc:6:21: warning: 'std::is_pod_v<A>' is deprecated: use is_standard_layout_v && is_trivial_v instead [-Wdeprecated-declarations]


>   inline constexpr bool is_pod_v = is_pod<_Tp>::value;
> template <typename _Tp>
>   inline constexpr bool is_literal_type_v = is_literal_type<_Tp>::value;
>diff --git a/libstdc++-v3/testsuite/20_util/is_pod/deprecated-2a.cc b/libstdc++-v3/testsuite/20_util/is_pod/deprecated-2a.cc
>new file mode 100644
>index 00000000000..1035a729434
>--- /dev/null
>+++ b/libstdc++-v3/testsuite/20_util/is_pod/deprecated-2a.cc
>@@ -0,0 +1,25 @@
>+// Copyright (C) 2019 Free Software Foundation, Inc.
>+//
>+// This file is part of the GNU ISO C++ Library.  This library is free
>+// software; you can redistribute it and/or modify it under the
>+// terms of the GNU General Public License as published by the
>+// Free Software Foundation; either version 3, or (at your option)
>+// any later version.
>+//
>+// This library is distributed in the hope that it will be useful,
>+// but WITHOUT ANY WARRANTY; without even the implied warranty of
>+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>+// GNU General Public License for more details.
>+//
>+// You should have received a copy of the GNU General Public License along
>+// with this library; see the file COPYING3.  If not see
>+// <http://www.gnu.org/licenses/>.
>+
>+// { dg-do compile { target c++2a } }

This test doesn't actually run:

UNSUPPORTED: 20_util/is_pod/deprecated-2a.cc

It needs { dg-options "-std=gnu++2a" }

>+
>+// { dg-prune-output "declared here" }
>+
>+#include <type_traits>
>+
>+static_assert(std::is_pod<int>::value); // { dg-warning "is deprecated" }
>+static_assert(std::is_pod_v<int>); // { dg-warning "is deprecated" }

I'll commit the attached patch after more testing.

N.B. it looks like the implementation of [[deprecated(msg)]] isn't
working quite right, because for is_pod I don't get the message:

pod.cc:6:21: warning: 'template<class _Tp> struct std::is_pod' is deprecated [-Wdeprecated-declarations]
     6 | static_assert( std::is_pod<A>::value );
       |                     ^~~~~~
In file included from pod.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/type_traits:693:5: note: declared here
   693 |     is_pod
       |     ^~~~~~

It works correctly for std::is_pod_v.

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