[libc++] basic_ios<wchar_t> cannot store fill character WCHAR_MAX

libcxx/docs/ReleaseNotes/19.rst

@@ -146,6 +146,13 @@ ABI Affecting Changes
   ``random_device`` could throw a ``system_error`` with this value. It now
   throws ``ENOMSG``.
 
+- libcxx ``std::basic_ios`` uses ``WEOF`` to indicate that the fill value is uninitialized. However, on platforms
+  where the size of ``char_type`` is equal to or greater than the size of ``int_type``,
+  ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish between ``WEOF`` and ``WCHAR_MAX``. Helper
+  class ``_OptionalFill`` is used for targets where a variable is needed to indicate whether the fill value
+  has been initialized. This is an ABI break on platforms where the size of ``char_type`` is equal to or greater
+  than the size of ``int_type``. Existing targets affected by this change can choose to keep the existing ABI
+  by undefining macro ``_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL``.
 
 Build System Changes
 --------------------

libcxx/include/__configuration/abi.h

@@ -91,6 +91,15 @@
 #  define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW
 // Dont' add an inline namespace for `std::filesystem`
 #    define _LIBCPP_ABI_NO_FILESYSTEM_INLINE_NAMESPACE
+// libcxx std::basic_ios uses WEOF to indicate that the fill value is
+// uninitialized. However, on platforms where the size of char_type is
+// equal to or greater than the size of int_type,
+// std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF
+// and WCHAR_MAX. Helper class _OptionalFill is used for targets where a
+// variable is needed to indicate whether the fill value has been initialized.
+// Existing targets where this would break ABI compatibility can choose to keep
+// the existing ABI by undefining macro _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL.
+#  define _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support
@@ -108,6 +117,10 @@
 #  if defined(__FreeBSD__) && __FreeBSD__ < 14
 #    define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #  endif
+// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+#  if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
+#    define _LIBCXX_IOS_FORCE_OPTIONAL_FILL
+#  endif
 #endif
 
 // We had some bugs where we use [[no_unique_address]] together with construct_at,

libcxx/include/ios

@@ -224,6 +224,7 @@ storage-class-specifier const error_category& iostream_category() noexcept;
 #include <__system_error/error_code.h>
 #include <__system_error/error_condition.h>
 #include <__system_error/system_error.h>
+#include <__type_traits/conditional.h>
 #include <__utility/swap.h>
 #include <__verbose_abort>
 #include <version>
@@ -521,6 +522,31 @@ inline _LIBCPP_HIDE_FROM_ABI void ios_base::exceptions(iostate __iostate) {
   clear(__rdstate_);
 }
 
+template <class _Traits>
+// Attribute 'packed' is used to keep the layout compatible with the previous
+// definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
+struct _LIBCPP_PACKED _OptionalFill {
+  _LIBCPP_HIDE_FROM_ABI _OptionalFill() : __set_(false) { }
+  _LIBCPP_HIDE_FROM_ABI _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
+  _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __set_; }
+  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
+
+private:
+  typename _Traits::int_type __fill_val_;
+  bool __set_;
+};
+
+template <class _Traits>
+struct _LIBCPP_PACKED _SentinelValueFill {
+  _LIBCPP_HIDE_FROM_ABI _SentinelValueFill() : __fill_val_(_Traits::eof()) { }
+  _LIBCPP_HIDE_FROM_ABI _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
+  _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __fill_val_ != _Traits::eof(); }
+  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
+
+private:
+  typename _Traits::int_type __fill_val_;
+};
+
 template <class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS basic_ios : public ios_base {
 public:
@@ -590,7 +616,19 @@ protected:
 
 private:
   basic_ostream<char_type, traits_type>* __tie_;
-  mutable int_type __fill_;
+
+#if defined(_LIBCXX_IOS_FORCE_OPTIONAL_FILL)
+  using _FillType = _OptionalFill<_Traits>;
+#elif defined(_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL)
+  using _FillType = _If<
+      sizeof(char_type) >= sizeof(int_type),
+      _OptionalFill<_Traits>,
+      _SentinelValueFill<_Traits>
+  >;
+#else
+  using _FillType = _SentinelValueFill<_Traits>;
+#endif
+  mutable _FillType __fill_;
 };
 
 template <class _CharT, class _Traits>
@@ -605,7 +643,7 @@ template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI void basic_ios<_CharT, _Traits>::init(basic_streambuf<char_type, traits_type>* __sb) {
   ios_base::init(__sb);
   __tie_  = nullptr;
-  __fill_ = traits_type::eof();
+  __fill_ = widen(' ');
 }
 
 template <class _CharT, class _Traits>
@@ -655,16 +693,16 @@ inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::widen(char __c)
 
 template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::fill() const {
-  if (traits_type::eq_int_type(traits_type::eof(), __fill_))
+  if (!__fill_.__is_set())
     __fill_ = widen(' ');
-  return __fill_;
+  return __fill_.__fill();
 }
 
 template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::fill(char_type __ch) {
-  if (traits_type::eq_int_type(traits_type::eof(), __fill_))
+  if (!__fill_.__is_set())
     __fill_ = widen(' ');
-  char_type __r = __fill_;
+  char_type __r = __fill_.__fill();
   __fill_       = __ch;
   return __r;
 }

libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Test that weof as a wchar_t value can be set as the fill character.
+
+// UNSUPPORTED: no-wide-characters
+// XFAIL: target=={{.+}}-windows-{{.+}} && stdlib=libc++ && libcpp-abi-version=1
+
+#include <iomanip>
+#include <ostream>
+#include <cassert>
+#include <string>
+
+template <class CharT>
+struct testbuf : public std::basic_streambuf<CharT> {
+  testbuf() {}
+};
+
+int main(int, char**) {
+  testbuf<wchar_t> sb;
+  std::wostream os(&sb);
+  os << std::setfill((wchar_t)std::char_traits<wchar_t>::eof());
+  assert(os.fill() == (wchar_t)std::char_traits<wchar_t>::eof());
+
+  return 0;
+}