[libcxx] Fix the error checking for wctob_l, fixing locale narrow function on Windows

[mstorsjo]

Summary

According to POSIX.1 (and Glibc docs, and Microsoft docs), the wctob
function returns EOF on error, not WEOF. (And wctob_l should consequently
do the same.)

The previous misconception about what this function returns on errors
seems to stem from incorrect documentation in macOS, stemming from BSD
docs with the same issue. The corresponding documentation bug in FreeBSD
was fixed in 2012 in
https://github.com/freebsd/freebsd-src/commit/945aab90991bdaeabeb6ef25112975a96c01dd4e,
but it hasn't been fixed for macOS yet.

The issue seems to only be a documentation issue; the implementation
on macOS actually does use EOF, not WEOF:
https://opensource.apple.com/source/Libc/Libc-1439.40.11/locale/FreeBSD/wctob.c.auto.html

On most Unices, EOF and WEOF are the same value, but on Windows,
EOF is -1, while WEOF is (unsigned short)0xFFFF. By fixing this,
two tests start passing on Windows.

[• Quuxplusone]

Looks like an improvement to me — but I'd like to see my comment explored a bit.

libcxx/src/locale.cpp

1527

I don't think the cast to int is doing anything anymore; what goes wrong if you remove it? Ditto below.

[mstorsjo]

libcxx/src/locale.cpp

1527

Sure, it should work just fine to drop the cast.

[mstorsjo]

Removed the now unused static_cast<int>().

[mstorsjo]

(The CI failure was spurious and went away when I retried that job, fwiw.)

[• Quuxplusone]

LGTM! One nit.

libcxx/src/locale.cpp

1527

Here and below, I'd now prefer parens
return (r != EOF) ? static_cast<char>(r) : dfault;
for clarity, although it's no big deal.

[mstorsjo]

libcxx/src/locale.cpp

1527

Ok, sure, will do. Yeah I prefer that form for clarity, too.

libcxx/src/locale.cpp

for (; low != high; ++low, ++dest)
         *dest = __libcpp_btowc_l(*low, __l);
     return low;
 }
 
 char
 ctype_byname<wchar_t>::do_narrow(char_type c, char dfault) const
 {
     int r = __libcpp_wctob_l(c, __l);
-    return r != static_cast<int>(WEOF) ? static_cast<char>(r) : dfault;
+    return (r != EOF) ? static_cast<char>(r) : dfault;
 }
 
 const wchar_t*
 ctype_byname<wchar_t>::do_narrow(const char_type* low, const char_type* high, char dfault, char* dest) const
 {
     for (; low != high; ++low, ++dest)
     {
         int r = __libcpp_wctob_l(*low, __l);
-        *dest = r != static_cast<int>(WEOF) ? static_cast<char>(r) : dfault;
+        *dest = (r != EOF) ? static_cast<char>(r) : dfault;
     }
     return low;
 }
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
 // template <> class codecvt<char, char, mbstate_t>
 
 locale::id codecvt<char, char, mbstate_t>::id;

libcxx/test/std/localization/locale.categories/category.ctype/locale.ctype.byname/narrow_1.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: locale.en_US.UTF-8
 // REQUIRES: locale.fr_CA.ISO8859-1
-// XFAIL: LIBCXX-WINDOWS-FIXME
 // XFAIL: libcpp-has-no-wide-characters
 
 // <locale>
 
 // template <class charT> class ctype_byname;
 
 // char narrow(charT c, char dfault) const;
 

libcxx/test/std/localization/locale.categories/category.ctype/locale.ctype.byname/narrow_many.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: locale.en_US.UTF-8
 // REQUIRES: locale.fr_CA.ISO8859-1
-// XFAIL: LIBCXX-WINDOWS-FIXME
 // XFAIL: libcpp-has-no-wide-characters
 
 // <locale>
 
 // template <class charT> class ctype_byname;
 
 // const charT* narrow(const charT* low, const charT*, char dfault, char* to) const;