Addressed comments.

-define and use macros _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL and _LIBCXX_IOS_FORCE_OPTIONAL_FILL.

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

@@ -617,21 +617,16 @@ protected:
 private:
   basic_ostream<char_type, traits_type>* __tie_;
 
-#if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
-// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+#if defined(_LIBCXX_IOS_FORCE_OPTIONAL_FILL)
   using _FillType = _OptionalFill<_Traits>;
-#else
-#if defined(_WIN32)
-  static const bool _OptOutForABICompat = true;
-#else
-  static const bool _OptOutForABICompat = false;
-#endif
-
+#elif defined(_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL)
   using _FillType = _If<
-      sizeof(char_type) >= sizeof(int_type) && !_OptOutForABICompat,
+      sizeof(char_type) >= sizeof(int_type),
       _OptionalFill<_Traits>,
       _SentinelValueFill<_Traits>
   >;
+#else
+  using _FillType = _SentinelValueFill<_Traits>;
 #endif
   mutable _FillType __fill_;
 };