[Patch][libstdc++] Add constexpr specifier to function

[Deev Patel]

Things like std::atomic are currently unable to be created in a
constexpr context with clang 18 and c++20. Example compilation error
```
external/com_google_tcmalloc/tcmalloc/parameters.cc:223:17: error: variable
does not have a constant initializer
  223 | Parameters::peak_sampling_heap_growth_fraction_(1.1);
  | ^~~~
external/com_google_tcmalloc/tcmalloc/parameters.cc:222:1: note: required
by 'constinit' specifier here
  222 | ABSL_CONST_INIT std::atomic
  | ^~~
external/com_google_absl/absl/base/attributes.h:743:25: note: expanded from
macro 'ABSL_CONST_INIT'
  743 | #define ABSL_CONST_INIT constinit
  | ^
external/local_config_clang/crosstool/extra_tools/lib/gcc/x86_64-linux-gnu/14.1.0/../../../../include/c++/14.1.0/bits/atomic_base.h:1286:9:
note: non-constexpr function '__clear_padding' cannot be used in a
constant expression
 1286 |   { __atomic_impl::__clear_padding(_M_fp); }
  | ^
external/local_config_clang/crosstool/extra_tools/lib/gcc/x86_64-linux-gnu/14.1.0/../../../../include/c++/14.1.0/atomic:1644:38:
note: in call to '__atomic_float(1.10e+00)'
 1644 |   atomic(double __fp) noexcept : __atomic_float(__fp)
```

This patch adds the necessary constexpr specifier.

- Deev Patel

diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index 062f1549740..5e89f66f620 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -968,7 +968,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }

 template
-  _GLIBCXX_ALWAYS_INLINE _Tp*
+  constexpr _GLIBCXX_ALWAYS_INLINE _Tp*
   __clear_padding(_Tp& __val) noexcept
   {
auto* __ptr = std::__addressof(__val);

[PATCH] libstdc++: Add missing constexpr specifier and function overloads

[Deev Patel via Gcc-patches]

Hi,

A couple of virtual functions in the libstdc++ format header are marked
constexpr in the base class, but not in the derived class. This was causing
build failures when trying to compile latest gcc libstd with clang 16 using
c++20. Adding the constexpr specifier resolves the issue.

2023-07-23  Deev Patel  

* include/std/format: Add missing constexpr specifiers on function 
overloads


>From ac34afa1109b4c82e5cc377f49abf55422b89529 Mon Sep 17 00:00:00 2001
From: Deev Patel 
Date: Sun, 23 Jul 2023 20:08:46 -0700
Subject: [PATCH] [libstdc++] Add missing constexpr specifiers on function
 overloads

---
 libstdc++-v3/include/std/format | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 9710bff3c03..0c6069b2681 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -3554,14 +3554,14 @@ namespace __format

   using iterator = typename _Scanner<_CharT>::iterator;

-  void
+  constexpr void
   _M_on_chars(iterator __last) override
   {
basic_string_view<_CharT> __str(this->begin(), __last);
_M_fc.advance_to(__format::__write(_M_fc.out(), __str));
   }

-  void
+  constexpr void
   _M_format_arg(size_t __id) override
   {
using _Context = basic_format_context<_Out, _CharT>;
-- 
2.41.0