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

[Giuseppe D'Angelo]

Hello,








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
	  char-like type.
	* include/std/string_view: port away from is_trivial.

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>);
       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

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).

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