[libcxx] Define _LIBCPP_HAS_C8RTOMB_MBRTOC8 to true if compiling with clang

Define `_LIBCPP_HAS_C8RTOMB_MBRTOC8` to `1` if compiling with clang.

Some tests involving functionality from `uchar.h`/`cuchar` fail when the
platform or the supporting C library does not provide support for the
corresponding features. These have been xfailed.

This patch will enable the adoption of newer picolibc versions.

Author: vhscampos

libcxx/include/__config

@@ -852,7 +852,9 @@ typedef __char32_t char32_t;
 // the latter depends on internal GNU libc details that are not appropriate
 // to depend on here, so any declarations present when __cpp_char8_t is not
 // defined are ignored.
-#  if defined(_LIBCPP_GLIBC_PREREQ)
+#  if defined(__clang__)
+#    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
+#  elif defined(_LIBCPP_GLIBC_PREREQ)
 #    if _LIBCPP_GLIBC_PREREQ(2, 36) && defined(__cpp_char8_t)
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
 #    else

libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp

@@ -23,11 +23,6 @@
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
-#if !defined(TEST_HAS_NO_C8RTOMB_MBRTOC8)
-ASSERT_SAME_TYPE(size_t, decltype(mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
-ASSERT_SAME_TYPE(size_t, decltype(c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
-#endif
-
 ASSERT_SAME_TYPE(size_t, decltype(mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(size_t, decltype(c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 

libcxx/test/std/depr/depr.c.headers/uchar_h_char8_t.compile.pass.cpp

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// The following platforms do not provide mbrtoc8 and c8rtomb so the tests fail
+// XFAIL: target={{.+}}-aix{{.*}}
+// XFAIL: android
+// XFAIL: darwin
+// XFAIL: freebsd
+// XFAIL: windows
+// XFAIL: glibc-no-char8_t-support
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
+// <uchar.h>
+
+#include <uchar.h>
+
+#include "test_macros.h"
+
+ASSERT_SAME_TYPE(size_t, decltype(mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(size_t, decltype(c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));

libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp

@@ -23,11 +23,6 @@
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
-#if !defined(TEST_HAS_NO_C8RTOMB_MBRTOC8)
-ASSERT_SAME_TYPE(std::size_t, decltype(std::mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
-ASSERT_SAME_TYPE(std::size_t, decltype(std::c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
-#endif
-
 ASSERT_SAME_TYPE(std::size_t, decltype(std::mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(std::size_t, decltype(std::c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 

libcxx/test/std/strings/c.strings/cuchar_char8_t.compile.pass.cpp

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// The following platforms do not provide mbrtoc8 and c8rtomb so the tests fail
+// XFAIL: target={{.+}}-aix{{.*}}
+// XFAIL: android
+// XFAIL: darwin
+// XFAIL: freebsd
+// XFAIL: windows
+// XFAIL: glibc-no-char8_t-support
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
+// <cuchar>
+
+#include <cuchar>
+
+#include "test_macros.h"
+
+ASSERT_SAME_TYPE(std::size_t, decltype(std::mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(std::size_t, decltype(std::c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));

libcxx/test/std/strings/c.strings/no_c8rtomb_mbrtoc8.verify.cpp

@@ -198,6 +198,23 @@ def _mingwSupportsModules(cfg):
           """,
         ),
     ),
+    # Check for Glibc < 2.36, where there was no support for char8_t functions
+    Feature(
+        name="glibc-no-char8_t-support",
+        when=lambda cfg: "__GLIBC__" in compilerMacros(cfg)
+        and not sourceBuilds(
+            cfg,
+            """
+            #include <uchar.h>
+            #include <wchar.h>
+            mbstate_t s;
+            int main(void) {
+                char8_t c;
+                return mbrtoc8(&c, "", 0, &s);
+            }
+            """,
+        ),
+    ),
     # Whether Bash can run on the executor.
     # This is not always the case, for example when running on embedded systems.
     #