Re: [PATCH 1/6] libstdc++: port away from is_trivial in string classes

[Jonathan Wakely]

On 09/12/24 13:06 +0100, Giuseppe D'Angelo wrote:

Hello,

This is a series of commits that will end up with the deprecation of the is_trivial type trait in C++26 (P3247R2).

Existing usages in libstdc+++ need to be ported away, either to the direct replacement (is_trivially_copyable && is_trivially_default_constructible) or, when possible, to more specific checks (as "is_trivially_copyable" is a check which suffers from the same flaws of "is_trivial").

I've split the work in multiple smaller commits for ease of review (and amendment in case I get something wrong).

Thanks, I'll amend the ChangeLog entry as noted below and then
test+push this. I'll review the rest of the patch series too.

When submitting patches please state how/where it was tested, just so
the mailing list archives have a record that it was tested, and on
which target. I typically just say "Tested x86_64-linux" as a
shorthand for x86_64-pc-linux-gnu because I'm too lazy to type it all,
and everybody knows what I mean.

Thanks,
--
Giuseppe D'Angelo

From 1dcceca767df8e403bffd82a0d5e08d343bf33c5 Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
Date: Mon, 9 Dec 2024 01:43:27 +0100
Subject: [PATCH 1/6] libstdc++: port away from is_trivial in string classes

In preparation for the deprecation of is_trivial (P3247R2), stop using
it from std::string_view. Also, add the same detection to std::string
(described in [strings.general]/2).

libstdc++-v3/ChangeLog:

        * include/bits/basic_string.h: add a static_assert on the

Capital letter for "Add".

          char-like type.

N.B. this should be aligned to the '*' not the 'include'.

        * include/std/string_view: port away from is_trivial.

Capital letter for "Port".

Signed-off-by: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
---
libstdc++-v3/include/bits/basic_string.h | 3 +++
libstdc++-v3/include/std/string_view     | 4 +++-
2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h 
b/libstdc++-v3/include/bits/basic_string.h
index 17b973c8b45..8369c24d3ae 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -88,6 +88,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
    class basic_string
    {
#if __cplusplus >= 202002L
+      static_assert(is_trivially_copyable_v<_CharT>
+         && is_trivially_default_constructible_v<_CharT>
+         && is_standard_layout_v<_CharT>);

This only enables the assertion for C++20 and later, but I think
that's fine.

      static_assert(is_same_v<_CharT, typename _Traits::char_type>);
      static_assert(is_same_v<_CharT, typename _Alloc::value_type>);
      using _Char_alloc_type = _Alloc;
diff --git a/libstdc++-v3/include/std/string_view 
b/libstdc++-v3/include/std/string_view
index 96350f96b3c..493edec26dc 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -108,7 +108,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    class basic_string_view
    {
      static_assert(!is_array_v<_CharT>);
-      static_assert(is_trivial_v<_CharT> && is_standard_layout_v<_CharT>);
+      static_assert(is_trivially_copyable_v<_CharT>
+         && is_trivially_default_constructible_v<_CharT>
+         && is_standard_layout_v<_CharT>);
      static_assert(is_same_v<_CharT, typename _Traits::char_type>);

    public:
--
2.34.1