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`libcxx std::basic_ios` uses `WEOF` to indicate the `fill` value is uninitialized. On some platforms (e.g AIX and zOS in 64-bit mode) `wchar_t` is 4 bytes `unsigned` and `wint_t` is also 4 bytes which means `WEOF` cannot be distinguished from `WCHAR_MAX` by `std::char_traits<wchar_t>::eq_int_type()`, meaning this valid character value cannot be stored on affected platforms (as the implementation triggers reinitialization to `widen(’ ’)`). This patch introduces a new helper class `_FillHelper` uses a boolean variable to indicate whether the fill character has been initialized, which is used by default in libcxx ABI version 2. The patch does not affect ABI version 1 except for targets AIX in 32- and 64-bit and z/OS in 64-bit (so that the layout of the implementation is compatible with the current IBM system provided libc++) This is a continuation of Phabricator patch [D124555](https://reviews.llvm.org/D124555). This patch uses a modified version of the [approach](https://reviews.llvm.org/D124555#3566746) suggested by @ldionne . --------- Co-authored-by: Louis Dionne <ldionne.2@gmail.com> Co-authored-by: David Tenty <daltenty.dev@gmail.com>