Added early initialization of codecvt error category.

Marked codecvt error category constructor constexpr and defined
it unconditionally to work around clang bug in XCode 8: the compiler
requires a user-defined constructor to initialize the static const object
of the category.

When constexpr is not available (more precisely, when it is not enabled
by Boost.System), try to dynamic-initialize the category instance early.
For MSVC, invoke codecvt_error_category() from the early global initialization
routine in path.cpp. For other compilers, use a global initializer, possibly
augmented with init_priority attribute.

Closes https://github.com/boostorg/filesystem/issues/227.

include/boost/filesystem/path_traits.hpp

@@ -29,7 +29,7 @@
 namespace boost {
 namespace filesystem {
 
-BOOST_FILESYSTEM_DECL const system::error_category& codecvt_error_category();
+BOOST_FILESYSTEM_DECL system::error_category const& codecvt_error_category() BOOST_NOEXCEPT;
 //  uses std::codecvt_base::result used for error codes:
 //
 //    ok:       Conversion successful.

src/codecvt_error_category.cpp

@@ -1,6 +1,7 @@
 //  codecvt_error_category implementation file  ----------------------------------------//
 
-//  Copyright Beman Dawes 2009
+//  Copyright 2009 Beman Dawes
+//  Copyright 2022 Andrey Semashev
 
 //  Distributed under the Boost Software License, Version 1.0.
 //  See http://www.boost.org/LICENSE_1_0.txt)
@@ -15,12 +16,11 @@
 
 #include <boost/filesystem/config.hpp>
 #include <boost/filesystem/path_traits.hpp>
-#include <boost/system/error_code.hpp>
+#include <boost/system/error_category.hpp>
 #include <locale>
 #include <string>
-#include <vector>
-#include <cstdlib>
-#include <cassert>
+
+#include "private_config.hpp"
 
 //--------------------------------------------------------------------------------------//
 
@@ -33,17 +33,13 @@ class codecvt_error_cat BOOST_FINAL :
     public boost::system::error_category
 {
 public:
-#if !defined(BOOST_CLANG) || __clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ > 8)
-    BOOST_DEFAULTED_FUNCTION(codecvt_error_cat(), {})
-#else
     // clang up to version 3.8 requires a user-defined default constructor in order to be able to declare a static constant of the error category.
-    codecvt_error_cat() {}
-#endif
-    const char* name() const BOOST_SYSTEM_NOEXCEPT BOOST_OVERRIDE;
+    BOOST_SYSTEM_CONSTEXPR codecvt_error_cat() BOOST_NOEXCEPT {}
+    const char* name() const BOOST_NOEXCEPT BOOST_OVERRIDE;
     std::string message(int ev) const BOOST_OVERRIDE;
 };
 
-const char* codecvt_error_cat::name() const BOOST_SYSTEM_NOEXCEPT
+const char* codecvt_error_cat::name() const BOOST_NOEXCEPT
 {
     return "codecvt";
 }
@@ -74,11 +70,36 @@ std::string codecvt_error_cat::message(int ev) const
 
 } // unnamed namespace
 
-BOOST_FILESYSTEM_DECL boost::system::error_category const& codecvt_error_category()
+BOOST_FILESYSTEM_DECL boost::system::error_category const& codecvt_error_category() BOOST_NOEXCEPT
 {
-    static const codecvt_error_cat codecvt_error_cat_const;
+    static
+#if defined(BOOST_SYSTEM_HAS_CONSTEXPR)
+        constexpr
+#else
+        const
+#endif
+        codecvt_error_cat codecvt_error_cat_const;
     return codecvt_error_cat_const;
 }
 
+// Try to initialize the error category instance as early as possible to make sure it is
+// available during global deinitialization stage. For MSVC, codecvt_error_category() will
+// be called early by MSVC-specific initialization routine in path.cpp.
+#if !defined(BOOST_SYSTEM_HAS_CONSTEXPR) && !defined(_MSC_VER)
+
+namespace {
+
+struct codecvt_error_category_initializer
+{
+    codecvt_error_category_initializer() { boost::filesystem::codecvt_error_category(); }
+};
+
+BOOST_FILESYSTEM_INIT_PRIORITY(BOOST_FILESYSTEM_PATH_GLOBALS_INIT_PRIORITY) BOOST_ATTRIBUTE_UNUSED BOOST_FILESYSTEM_ATTRIBUTE_RETAIN
+const codecvt_error_category_initializer g_codecvt_error_category_initializer;
+
+} // namespace
+
+#endif // !defined(BOOST_SYSTEM_HAS_CONSTEXPR) && !defined(_MSC_VER)
+
 } // namespace filesystem
 } // namespace boost

src/path.cpp

@@ -12,8 +12,9 @@
 
 #include <boost/filesystem/config.hpp>
 #include <boost/filesystem/path.hpp>
+#include <boost/filesystem/path_traits.hpp> // codecvt_error_category()
 #include <boost/scoped_array.hpp>
-#include <boost/system/error_code.hpp>
+#include <boost/system/error_category.hpp> // for BOOST_SYSTEM_HAS_CONSTEXPR
 #include <boost/assert.hpp>
 #include <algorithm>
 #include <iterator>
@@ -42,7 +43,6 @@
 namespace fs = boost::filesystem;
 
 using boost::filesystem::path;
-using boost::system::error_code;
 
 //--------------------------------------------------------------------------------------//
 //                                                                                      //
@@ -1514,6 +1514,10 @@ void __cdecl destroy_path_globals()
 
 BOOST_FILESYSTEM_INIT_FUNC init_path_globals()
 {
+#if !defined(BOOST_SYSTEM_HAS_CONSTEXPR)
+    // codecvt_error_category needs to be called early to dynamic-initialize the error category instance
+    boost::filesystem::codecvt_error_category();
+#endif
     std::atexit(&destroy_path_globals);
     return BOOST_FILESYSTEM_INITRETSUCCESS_V;
 }