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

libcxx/docs/ReleaseNotes/19.rst

@@ -146,6 +146,15 @@ 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`` and ``char_type`` is
+  unsigned, ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish
+  between ``WEOF`` and ``WCHAR_MAX``. Helper class ``_FillHelper`` is
+  added where a boolean variable is used to indicate whether the fill value
+  has been initialized so that a fill value ``WEOF`` set by the user won't be
+  treated as indicating the fill value is uninitialized. ``_FillHelper`` is
+  used in ABI version 2 and does not affect ABI version 1.
 
 Build System Changes
 --------------------

libcxx/include/__configuration/abi.h

@@ -91,6 +91,14 @@
 #  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 and char_type is unsigned,
+// std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF
+// and WCHAR_MAX. New helper class _FillHelper uses a boolean variable to indicate
+// whether the fill value has been initialized so that a fill value WEOF set
+// by the user won't be treated as indicating the fill value is uninitialized.
+#  define _LIBCXX_IOS_USE_FILL_HELPER
 #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 +116,10 @@
 #  if defined(__FreeBSD__) && __FreeBSD__ < 14
 #    define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #  endif
+// AIX and 64-bit MVS must use _FillHelper for ABI backward compatibility.
+#  if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
+#    define _LIBCXX_IOS_USE_FILL_HELPER
+#  endif
 #endif
 
 // We had some bugs where we use [[no_unique_address]] together with construct_at,

libcxx/include/ios

@@ -521,6 +521,31 @@ 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 _FillHelper {
+  _LIBCPP_HIDE_FROM_ABI void __init() { __set_ = false; }
+  _LIBCPP_HIDE_FROM_ABI _FillHelper& 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 void __init() { __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 +615,13 @@ protected:
 
 private:
   basic_ostream<char_type, traits_type>* __tie_;
-  mutable int_type __fill_;
+
+#if defined(_LIBCXX_IOS_USE_FILL_HELPER)
+  using _FillType = _FillHelper<_Traits>;
+#else
+  using _FillType = _SentinelValueFill<_Traits>;
+#endif
+  mutable _FillType __fill_;
 };
 
 template <class _CharT, class _Traits>
@@ -605,7 +636,7 @@ 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_.__init();
 }
 
 template <class _CharT, class _Traits>
@@ -655,16 +686,16 @@ 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
+// REQUIRES: target=powerpc{{(64)?}}-ibm-aix || target=s390x-ibm-zos || libcpp-abi-version=2
+
+#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;
+}