[libc++] Reduce the dependency of the locale base API on the base system from the headers

Many parts of the locale base API are only required when building the
shared/static library, but not from the headers. Document those functions
and carve out a few of those that don't work when _XOPEN_SOURCE is defined
to something old.

Fixes #117630

Author: ldionne

libcxx/include/__locale_dir/locale_base_api.h

@@ -23,12 +23,16 @@
 // Variadic functions may be implemented as templates with a parameter pack instead
 // of C-style variadic functions.
 //
+// Most of these functions are only required when building the library. Functions that are also
+// required when merely using the headers are marked as such below.
+//
 // TODO: __localeconv shouldn't take a reference, but the Windows implementation doesn't allow copying __locale_t
+// TODO: Eliminate the need for any of these functions from the headers.
 //
 // Locale management
 // -----------------
 // namespace __locale {
-//  using __locale_t = implementation-defined;
+//  using __locale_t = implementation-defined;  // required by the headers
 //  using __lconv_t  = implementation-defined;
 //  __locale_t  __newlocale(int, const char*, __locale_t);
 //  void        __freelocale(__locale_t);
@@ -60,8 +64,8 @@
 // namespace __locale {
 //  int     __islower(int, __locale_t);
 //  int     __isupper(int, __locale_t);
-//  int     __isdigit(int, __locale_t);
-//  int     __isxdigit(int, __locale_t);
+//  int     __isdigit(int, __locale_t);  // required by the headers
+//  int     __isxdigit(int, __locale_t); // required by the headers
 //  int     __toupper(int, __locale_t);
 //  int     __tolower(int, __locale_t);
 //  int     __strcoll(const char*, const char*, __locale_t);
@@ -99,9 +103,10 @@
 //  int     __mbtowc(wchar_t*, const char*, size_t, __locale_t);
 //  size_t  __mbrlen(const char*, size_t, mbstate_t*, __locale_t);
 //  size_t  __mbsrtowcs(wchar_t*, const char**, size_t, mbstate_t*, __locale_t);
-//  int     __snprintf(char*, size_t, __locale_t, const char*, ...);
-//  int     __asprintf(char**, __locale_t, const char*, ...);
-//  int     __sscanf(const char*, __locale_t, const char*, ...);
+//
+//  int     __snprintf(char*, size_t, __locale_t, const char*, ...); // required by the headers
+//  int     __asprintf(char**, __locale_t, const char*, ...);        // required by the headers
+//  int     __sscanf(const char*, __locale_t, const char*, ...);     // required by the headers
 // }
 
 #if defined(__APPLE__)

libcxx/include/__locale_dir/support/bsd_like.h

@@ -160,19 +160,23 @@ inline _LIBCPP_HIDE_FROM_ABI wint_t __btowc(int __c, __locale_t __loc) { return
 
 inline _LIBCPP_HIDE_FROM_ABI int __wctob(wint_t __c, __locale_t __loc) { return ::wctob_l(__c, __loc); }
 
+#  ifdef _LIBCPP_BUILDING_LIBRARY
 inline _LIBCPP_HIDE_FROM_ABI size_t
 __wcsnrtombs(char* __dest, const wchar_t** __src, size_t __nwc, size_t __len, mbstate_t* __ps, __locale_t __loc) {
   return ::wcsnrtombs_l(__dest, __src, __nwc, __len, __ps, __loc);
 }
+#  endif
 
 inline _LIBCPP_HIDE_FROM_ABI size_t __wcrtomb(char* __s, wchar_t __wc, mbstate_t* __ps, __locale_t __loc) {
   return ::wcrtomb_l(__s, __wc, __ps, __loc);
 }
 
+#  ifdef _LIBCPP_BUILDING_LIBRARY
 inline _LIBCPP_HIDE_FROM_ABI size_t
 __mbsnrtowcs(wchar_t* __dest, const char** __src, size_t __nms, size_t __len, mbstate_t* __ps, __locale_t __loc) {
   return ::mbsnrtowcs_l(__dest, __src, __nms, __len, __ps, __loc);
 }
+#  endif
 
 inline _LIBCPP_HIDE_FROM_ABI size_t
 __mbrtowc(wchar_t* __pwc, const char* __s, size_t __n, mbstate_t* __ps, __locale_t __loc) {

libcxx/test/libcxx/xopen_source.gen.py

@@ -0,0 +1,42 @@
+# ===----------------------------------------------------------------------===##
+#
+# 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
+#
+# ===----------------------------------------------------------------------===##
+
+# Make sure that libc++ headers work when defining _XOPEN_SOURCE=500.
+# We may not want to guarantee this forever, but since this works today and
+# it's something that users rely on, it makes sense to put a test on it.
+#
+# https://github.com/llvm/llvm-project/issues/117630
+
+# RUN: %{python} %s %{libcxx-dir}/utils
+
+import sys
+
+sys.path.append(sys.argv[1])
+from libcxx.header_information import (
+    lit_header_restrictions,
+    lit_header_undeprecations,
+    public_headers,
+)
+
+for header in public_headers:
+    for version in (500, 600, 700):
+        # TODO: <fstream> currently uses ::fseeko unguarded, which fails with _XOPEN_SOURCE=500.
+        if header == "fstream" and version == 500:
+            continue
+
+        print(
+            f"""\
+//--- {header}.xopen_source_{version}.compile.pass.cpp
+{lit_header_restrictions.get(header, '')}
+{lit_header_undeprecations.get(header, '')}
+
+// ADDITIONAL_COMPILE_FLAGS: -D_XOPEN_SOURCE={version}
+
+#include <{header}>
+"""
+        )