[gcc r12-6265] libstdc++: Improvements to standard error category objects (part deux)

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:096228d84e9238d97fe115623126373f5b67bdc1

commit r12-6265-g096228d84e9238d97fe115623126373f5b67bdc1
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 22 13:56:21 2021 +0100

    libstdc++: Improvements to standard error category objects (part deux)
    
    In r12-3860 the error categories in <system_error> were made final and
    immortal, but I missed the categories for <future> and <ios>. This makes
    the same changes to those.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++11/cxx11-ios_failure.cc (io_error_category): Define
            class and virtual functions as 'final'.
            (io_category_instance): Use constinit union to make the object
            immortal.
            * src/c++11/future.cc (future_error_category): Define class and
            virtual functions as 'final'.
            (future_category_instance): Use constinit union.

Diff:
---
 libstdc++-v3/src/c++11/cxx11-ios_failure.cc | 25 +++++++++++++++----------
 libstdc++-v3/src/c++11/future.cc            | 26 ++++++++++++++++----------
 2 files changed, 31 insertions(+), 20 deletions(-)

diff --git a/libstdc++-v3/src/c++11/cxx11-ios_failure.cc b/libstdc++-v3/src/c++11/cxx11-ios_failure.cc
index 5a151b6ec5c..ba4b1413bf9 100644
--- a/libstdc++-v3/src/c++11/cxx11-ios_failure.cc
+++ b/libstdc++-v3/src/c++11/cxx11-ios_failure.cc
@@ -44,14 +44,15 @@
 
 namespace
 {
-  struct io_error_category : std::error_category
+  struct io_error_category final : std::error_category
   {
-    virtual const char*
-    name() const noexcept
+    const char*
+    name() const noexcept final
     { return "iostream"; }
 
     _GLIBCXX_DEFAULT_ABI_TAG
-    virtual std::string message(int __ec) const
+    std::string
+    message(int __ec) const final
     {
       std::string __msg;
       switch (std::io_errc(__ec))
@@ -67,13 +68,17 @@ namespace
     }
   };
 
-  const io_error_category&
-  __io_category_instance() noexcept
+  struct constant_init
   {
-    static const io_error_category __ec{};
-    return __ec;
-  }
+    union {
+      unsigned char unused;
+      io_error_category cat;
+    };
+    constexpr constant_init() : cat() { }
+    ~constant_init() { /* do nothing, union member is not destroyed */ }
+  };
 
+  __constinit constant_init io_category_instance{};
 } // namespace
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -82,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   const error_category&
   iostream_category() noexcept
-  { return __io_category_instance(); }
+  { return io_category_instance.cat; }
 
   ios_base::failure::failure(const string& __str)
   : system_error(io_errc::stream, __str) { }
diff --git a/libstdc++-v3/src/c++11/future.cc b/libstdc++-v3/src/c++11/future.cc
index c5423931eb3..488ff17a1e6 100644
--- a/libstdc++-v3/src/c++11/future.cc
+++ b/libstdc++-v3/src/c++11/future.cc
@@ -27,14 +27,15 @@
 
 namespace
 {
-  struct future_error_category : public std::error_category
+  struct future_error_category final : public std::error_category
   {
-    virtual const char*
-    name() const noexcept
+    const char*
+    name() const noexcept final
     { return "future"; }
 
     _GLIBCXX_DEFAULT_ABI_TAG
-    virtual std::string message(int __ec) const
+    std::string
+    message(int __ec) const final
     {
       std::string __msg;
       switch (std::future_errc(__ec))
@@ -59,12 +60,17 @@ namespace
     }
   };
 
-  const future_error_category&
-  __future_category_instance() noexcept
+  struct constant_init
   {
-    static const future_error_category __fec{};
-    return __fec;
-  }
+    union {
+      unsigned char unused;
+      future_error_category cat;
+    };
+    constexpr constant_init() : cat() { }
+    ~constant_init() { /* do nothing, union member is not destroyed */ }
+  };
+
+  __constinit constant_init future_category_instance{};
 }
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -76,7 +82,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   { _GLIBCXX_THROW_OR_ABORT(future_error(make_error_code(future_errc(__i)))); }
 
   const error_category& future_category() noexcept
-  { return __future_category_instance(); }
+  { return future_category_instance.cat; }
 
   future_error::~future_error() noexcept { }