[gcc r12-6769] libstdc++: Use Clang attribute instead of __constinit

[gcc r12-6769] libstdc++: Use Clang attribute instead of __constinit

[Jonathan Wakely]

https://gcc.gnu.org/g:109f8af3d3c5424f103ceef5d77f5a447fb931ce

commit r12-6769-g109f8af3d3c5424f103ceef5d77f5a447fb931ce
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jan 20 11:18:34 2022 +0000

    libstdc++: Use Clang attribute instead of __constinit
    
    Clang doesn't support the __constinit extension that we use pre-C++20,
    but it does have its own equivalent attribute that can be used instead.
    
    This makes it a little easier to use Clang to build libstdc++ (which
    isn't supported. but is sometimes attempted for esoteric targets).
    
    libstdc++-v3/ChangeLog:
    
            * src/c++11/cxx11-ios_failure.cc (__constinit): Define as
            equivalent attribute for Clang.
            * src/c++11/future.cc (__constinit): Likewise.
            * src/c++11/system_error.cc (__constinit): Likewise.
            * src/c++17/memory_resource.cc (__constinit): Likewise.

Diff:
---
 libstdc++-v3/src/c++11/cxx11-ios_failure.cc | 4 ++++
 libstdc++-v3/src/c++11/future.cc            | 4 ++++
 libstdc++-v3/src/c++11/system_error.cc      | 4 ++++
 libstdc++-v3/src/c++17/memory_resource.cc   | 4 ++++
 4 files changed, 16 insertions(+)

diff --git a/libstdc++-v3/src/c++11/cxx11-ios_failure.cc b/libstdc++-v3/src/c++11/cxx11-ios_failure.cc
index ba4b1413bf9..14a7f9cfcb1 100644
--- a/libstdc++-v3/src/c++11/cxx11-ios_failure.cc
+++ b/libstdc++-v3/src/c++11/cxx11-ios_failure.cc
@@ -42,6 +42,10 @@
 # error This file should not be compiled for this configuration.
 #endif
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace
 {
   struct io_error_category final : std::error_category
diff --git a/libstdc++-v3/src/c++11/future.cc b/libstdc++-v3/src/c++11/future.cc
index 488ff17a1e6..c52c057ba1d 100644
--- a/libstdc++-v3/src/c++11/future.cc
+++ b/libstdc++-v3/src/c++11/future.cc
@@ -25,6 +25,10 @@
 #include <future>
 #include <bits/functexcept.h>
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace
 {
   struct future_error_category final : public std::error_category
diff --git a/libstdc++-v3/src/c++11/system_error.cc b/libstdc++-v3/src/c++11/system_error.cc
index 789f2b45382..7b1a5a20637 100644
--- a/libstdc++-v3/src/c++11/system_error.cc
+++ b/libstdc++-v3/src/c++11/system_error.cc
@@ -37,6 +37,10 @@
 #include <windows.h>
 #endif
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace
 {
   using std::string;
diff --git a/libstdc++-v3/src/c++17/memory_resource.cc b/libstdc++-v3/src/c++17/memory_resource.cc
index 5cdb35df2cd..bb6334c9694 100644
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/libstdc++-v3/src/c++17/memory_resource.cc
@@ -32,6 +32,10 @@
 # include <bits/move.h>		// std::__exchange
 #endif
 
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION