diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -490,6 +490,14 @@
 #    define _LIBCPP_SHORT_WCHAR 1
 #  endif
 
+#  if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
+// AIX and 64-bit MVS must use structure _OptionalFill in basic_ios for compatibility.
+#    define _LIBCPP_BASIC_IOS_MUST_USE_OPTIONAL_FILL 1
+#  elif defined(_WIN32) || defined(__APPLE__) || defined(__sun__) || defined(__linux__) || defined(__FreeBSD__) ||     \
+        defined(__NetBSD__) || defined(__OpenBSD__)
+#    define _LIBCPP_BASIC_IOS_MUST_USE_OPTIONAL_FILL 0
+#  endif
+
 // Libc++ supports various implementations of std::random_device.
 //
 // _LIBCPP_USING_DEV_RANDOM
diff --git a/libcxx/include/ios b/libcxx/include/ios
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -224,6 +224,7 @@
 #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>
@@ -605,6 +606,31 @@
     clear(__rdstate_);
 }
 
+template <class _Traits>
+// Attribute 'packed' is used to keep the layout compatible with the previous
+// definition of '__set_' in basic_ios on AIX. See D124555 for details.
+struct _LIBCPP_PACKED _OptionalFill {
+    _OptionalFill() : __set_(false) { }
+    _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
+    bool __is_set() const { return __set_; }
+    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 {
+    _SentinelValueFill() : __fill_val_(_Traits::eof()) { }
+    _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
+    bool __is_set() const { return __fill_val_ != _Traits::eof(); }
+    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
@@ -691,7 +717,23 @@
     void set_rdbuf(basic_streambuf<char_type, traits_type>* __sb);
 private:
     basic_ostream<char_type, traits_type>* __tie_;
-    mutable int_type __fill_;
+
+#if _LIBCPP_BASIC_IOS_MUST_USE_OPTIONAL_FILL
+    using _FillType = _OptionalFill<_Traits>;
+#else
+#if defined(_WIN32)
+    static const bool _OptOutForABICompat = true;
+#else
+    static const bool _OptOutForABICompat = false;
+#endif
+
+    using _FillType = _If<
+        sizeof(char_type) >= sizeof(int_type) && !_OptOutForABICompat,
+        _OptionalFill<_Traits>,
+        _SentinelValueFill<_Traits>
+    >;
+#endif
+    mutable _FillType __fill_;
 };
 
 template <class _CharT, class _Traits>
@@ -713,7 +755,7 @@
 {
     ios_base::init(__sb);
     __tie_ = nullptr;
-    __fill_ = traits_type::eof();
+    __fill_ = widen(' ');
 }
 
 template <class _CharT, class _Traits>
@@ -785,9 +827,9 @@
 _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>
@@ -795,9 +837,9 @@
 _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;
 }
diff --git a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
new file mode 100644
--- /dev/null
+++ b/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
+
+#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;
+}