commit 3d40bc9bf5c79343ea5a6cc355539542f4b56c9b
Author: Tom Honermann <tom@honermann.net>
Date:   Sat Jan 1 17:26:31 2022 -0500

    P0482R6 char8_t: declare std::c8rtomb and std::mbrtoc8 if provided by the C library.
    
    This change completes implementation of the C++20 proposal P0482R6 by
    adding declarations of std::c8rtomb() and std::mbrtoc8() if provided
    by the C library.
    
    Autoconf changes determine if the C library declares c8rtomb and mbrtoc8
    at global scope when uchar.h is included and compiled with either -fchar8_t
    or -std=c++20 enabled; new _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T and
    _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20 configuration macros are defined
    accordingly. The <cuchar> header declares these functions in the std
    namespace only if available and the _GLIBCXX_USE_CHAR8_T configuration
    macro is defined (by default it is defined if the C++20 __cpp_char8_t
    feature test macro is defined).
    
    New tests validate the presence of these declarations. The tests pass
    trivially if the C library does not provide these functions. Otherwise they
    ensure that the functions are declared when <cuchar> is included and
    either -fchar8_t or -std=c++20 is enabled.

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 635168d7e25..85235005c7e 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -2039,6 +2039,50 @@ AC_DEFUN([GLIBCXX_CHECK_UCHAR_H], [
 	      namespace std in <cuchar>.])
   fi
 
+  CXXFLAGS="$CXXFLAGS -fchar8_t"
+  if test x"$ac_has_uchar_h" = x"yes"; then
+    AC_MSG_CHECKING([for c8rtomb and mbrtoc8 in <uchar.h> with -fchar8_t])
+    AC_TRY_COMPILE([#include <uchar.h>
+		    namespace test
+		    {
+		      using ::c8rtomb;
+		      using ::mbrtoc8;
+		    }
+		   ],
+		   [], [ac_uchar_c8rtomb_mbrtoc8_fchar8_t=yes],
+		       [ac_uchar_c8rtomb_mbrtoc8_fchar8_t=no])
+  else
+    ac_uchar_c8rtomb_mbrtoc8_fchar8_t=no
+  fi
+  AC_MSG_RESULT($ac_uchar_c8rtomb_mbrtoc8_fchar8_t)
+  if test x"$ac_uchar_c8rtomb_mbrtoc8_fchar8_t" = x"yes"; then
+    AC_DEFINE(_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T, 1,
+	      [Define if c8rtomb and mbrtoc8 functions in <uchar.h> should be
+	      imported into namespace std in <cuchar> for -fchar8_t.])
+  fi
+
+  CXXFLAGS="$CXXFLAGS -std=c++20"
+  if test x"$ac_has_uchar_h" = x"yes"; then
+    AC_MSG_CHECKING([for c8rtomb and mbrtoc8 in <uchar.h> with -std=c++20])
+    AC_TRY_COMPILE([#include <uchar.h>
+		    namespace test
+		    {
+		      using ::c8rtomb;
+		      using ::mbrtoc8;
+		    }
+		   ],
+		   [], [ac_uchar_c8rtomb_mbrtoc8_cxx20=yes],
+		       [ac_uchar_c8rtomb_mbrtoc8_cxx20=no])
+  else
+    ac_uchar_c8rtomb_mbrtoc8_cxx20=no
+  fi
+  AC_MSG_RESULT($ac_uchar_c8rtomb_mbrtoc8_cxx20)
+  if test x"$ac_uchar_c8rtomb_mbrtoc8_cxx20" = x"yes"; then
+    AC_DEFINE(_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20, 1,
+	      [Define if c8rtomb and mbrtoc8 functions in <uchar.h> should be
+	      imported into namespace std in <cuchar> for C++20.])
+  fi
+
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
 ])
diff --git a/libstdc++-v3/config.h.in b/libstdc++-v3/config.h.in
index 10675fe2530..e09744961d1 100644
--- a/libstdc++-v3/config.h.in
+++ b/libstdc++-v3/config.h.in
@@ -1000,6 +1000,14 @@
 /* Define if obsolescent tmpnam is available in <stdio.h>. */
 #undef _GLIBCXX_USE_TMPNAM
 
+/* Define if c8rtomb and std functions in <uchar.h> should be
+   imported into namespace std in <cuchar> for -fchar8_t. */
+#undef _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T
+
+/* Define if c8rtomb and std functions in <uchar.h> should be
+   imported into namespace std in <cuchar> for C++20. */
+#undef _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20
+
 /* Define if utime is available in <utime.h>. */
 #undef _GLIBCXX_USE_UTIME
 
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index b1a0157d0b9..fae78ec1cc6 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -19130,6 +19130,82 @@ $as_echo "#define _GLIBCXX_USE_C11_UCHAR_CXX11 1" >>confdefs.h
 
   fi
 
+  CXXFLAGS="$CXXFLAGS -fchar8_t"
+  if test x"$ac_has_uchar_h" = x"yes"; then
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for c8rtomb and mbrtoc8 in <uchar.h> with -fchar8_t" >&5
+$as_echo_n "checking for c8rtomb and mbrtoc8 in <uchar.h> with -fchar8_t... " >&6; }
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <uchar.h>
+		    namespace test
+		    {
+		      using ::c8rtomb;
+		      using ::mbrtoc8;
+		    }
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_uchar_c8rtomb_mbrtoc8_fchar8_t=yes
+else
+  ac_uchar_c8rtomb_mbrtoc8_fchar8_t=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  else
+    ac_uchar_c8rtomb_mbrtoc8_fchar8_t=no
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_uchar_c8rtomb_mbrtoc8_fchar8_t" >&5
+$as_echo "$ac_uchar_c8rtomb_mbrtoc8_fchar8_t" >&6; }
+  if test x"$ac_uchar_c8rtomb_mbrtoc8_fchar8_t" = x"yes"; then
+
+$as_echo "#define _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T 1" >>confdefs.h
+
+  fi
+
+  CXXFLAGS="$CXXFLAGS -std=c++20"
+  if test x"$ac_has_uchar_h" = x"yes"; then
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for c8rtomb and mbrtoc8 in <uchar.h> with -std=c++20" >&5
+$as_echo_n "checking for c8rtomb and mbrtoc8 in <uchar.h> with -std=c++20... " >&6; }
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <uchar.h>
+		    namespace test
+		    {
+		      using ::c8rtomb;
+		      using ::mbrtoc8;
+		    }
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_uchar_c8rtomb_mbrtoc8_cxx20=yes
+else
+  ac_uchar_c8rtomb_mbrtoc8_cxx20=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  else
+    ac_uchar_c8rtomb_mbrtoc8_cxx20=no
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_uchar_c8rtomb_mbrtoc8_cxx20" >&5
+$as_echo "$ac_uchar_c8rtomb_mbrtoc8_cxx20" >&6; }
+  if test x"$ac_uchar_c8rtomb_mbrtoc8_cxx20" = x"yes"; then
+
+$as_echo "#define _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20 1" >>confdefs.h
+
+  fi
+
   CXXFLAGS="$ac_save_CXXFLAGS"
   ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
diff --git a/libstdc++-v3/include/c_compatibility/uchar.h b/libstdc++-v3/include/c_compatibility/uchar.h
index 1fe8a22f78a..b2346e70d2b 100644
--- a/libstdc++-v3/include/c_compatibility/uchar.h
+++ b/libstdc++-v3/include/c_compatibility/uchar.h
@@ -33,6 +33,14 @@
 
 #ifdef _GLIBCXX_NAMESPACE_C
 
+#if (_GLIBCXX_USE_CHAR8_T \
+     && (_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T \
+	 || (__cplusplus >= 202002 \
+	     && _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20)))
+using std::mbrtoc8;
+using std::c8rtomb;
+#endif // _GLIBCXX_USE_CHAR8_T
+
 #if _GLIBCXX_USE_C11_UCHAR_CXX11
 using std::mbrtoc16;
 using std::c16rtomb;
diff --git a/libstdc++-v3/include/c_global/cuchar b/libstdc++-v3/include/c_global/cuchar
index 57047d74218..943d2727878 100644
--- a/libstdc++-v3/include/c_global/cuchar
+++ b/libstdc++-v3/include/c_global/cuchar
@@ -48,10 +48,41 @@
 #include <bits/c++config.h>
 #include <cwchar>
 
-#if _GLIBCXX_USE_C11_UCHAR_CXX11
+#if (_GLIBCXX_USE_C11_UCHAR_CXX11 \
+     || (_GLIBCXX_USE_CHAR8_T \
+	 && (_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T \
+	     || (__cplusplus >= 202002 \
+		 && _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20))))
 
 #include <uchar.h>
 
+#endif
+
+
+// Support for mbrtoc8 and c8rtomb is conditioned on support by the C library.
+#if (_GLIBCXX_USE_CHAR8_T \
+     && (_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T \
+	 || (__cplusplus >= 202002 \
+	     && _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20)))
+
+#undef mbrtoc8
+#undef c8rtomb
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  using ::mbrtoc8;
+  using ::c8rtomb;
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+
+#endif // _GLIBCXX_USE_CHAR8_T
+
+
+#if _GLIBCXX_USE_C11_UCHAR_CXX11
+
 // Get rid of those macros defined in <uchar.h> in lieu of real functions.
 #undef mbrtoc16
 #undef c16rtomb
diff --git a/libstdc++-v3/include/c_std/cuchar b/libstdc++-v3/include/c_std/cuchar
index 7ce413e1bb0..56c11f7d24f 100644
--- a/libstdc++-v3/include/c_std/cuchar
+++ b/libstdc++-v3/include/c_std/cuchar
@@ -48,10 +48,42 @@
 #include <bits/c++config.h>
 #include <cwchar>
 
-#if _GLIBCXX_USE_C11_UCHAR_CXX11
+#if (_GLIBCXX_USE_C11_UCHAR_CXX11 \
+     || (_GLIBCXX_USE_CHAR8_T \
+	 && (_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T \
+	     || (__cplusplus >= 202002 \
+		 && _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20))))
 
 #include <uchar.h>
 
+#endif
+
+
+// Support for mbrtoc8 and c8rtomb is conditioned on support by the C library.
+#if (_GLIBCXX_USE_CHAR8_T \
+     && (_GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T \
+	 || (__cplusplus >= 202002 \
+	     && _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20)))
+
+// Get rid of those macros defined in <uchar.h> in lieu of real functions.
+#undef mbrtoc8
+#undef c8rtomb
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  using ::mbrtoc8;
+  using ::c8rtomb;
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+
+#endif // _GLIBCXX_USE_CHAR8_T
+
+
+#if _GLIBCXX_USE_C11_UCHAR_CXX11
+
 // Get rid of those macros defined in <uchar.h> in lieu of real functions.
 #undef mbrtoc16
 #undef c16rtomb
diff --git a/libstdc++-v3/testsuite/21_strings/headers/cuchar/functions_std_cxx20.cc b/libstdc++-v3/testsuite/21_strings/headers/cuchar/functions_std_cxx20.cc
new file mode 100644
index 00000000000..7c152ed42b5
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/headers/cuchar/functions_std_cxx20.cc
@@ -0,0 +1,12 @@
+// { dg-do compile }
+// { dg-options "-std=c++20" }
+
+#include <cuchar>
+
+namespace gnu
+{
+#if _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20
+  using std::mbrtoc8;
+  using std::c8rtomb;
+#endif // _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20
+}
diff --git a/libstdc++-v3/testsuite/21_strings/headers/cuchar/functions_std_fchar8_t.cc b/libstdc++-v3/testsuite/21_strings/headers/cuchar/functions_std_fchar8_t.cc
new file mode 100644
index 00000000000..1cfaf7427e5
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/headers/cuchar/functions_std_fchar8_t.cc
@@ -0,0 +1,12 @@
+// { dg-do compile }
+// { dg-options "-fchar8_t" }
+
+#include <cuchar>
+
+namespace gnu
+{
+#if _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T
+  using std::mbrtoc8;
+  using std::c8rtomb;
+#endif // _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T
+}