[libc++][CI] Reenables clang-tidy.

This is currently a test patch; locally clang-tidy seems to be used in
the Docker image.

libcxx/modules/std/chrono.inc

@@ -213,11 +213,18 @@ export namespace std {
     using std::chrono::ambiguous_local_time;
     using std::chrono::nonexistent_local_time;
 #    endif // if 0
+#  endif   // _LIBCPP_ENABLE_EXPERIMENTAL
+#endif     //  !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) &&
+           //  !defined(_LIBCPP_HAS_NO_LOCALIZATION)
 
     // [time.zone.info], information classes
     using std::chrono::local_info;
     using std::chrono::sys_info;
 
+#if !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) &&                              \
+    !defined(_LIBCPP_HAS_NO_LOCALIZATION)
+
+#  ifdef _LIBCPP_ENABLE_EXPERIMENTAL
 #    if 0
     // [time.zone.timezone], class time_zone
     using std::chrono::choose;
@@ -248,7 +255,7 @@ export namespace std {
 #    endif
 #  endif // _LIBCPP_ENABLE_EXPERIMENTAL
 #endif   //  !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) &&
-         //    !defined(_LIBCPP_HAS_NO_LOCALIZATION)
+         //  !defined(_LIBCPP_HAS_NO_LOCALIZATION)
 
   } // namespace chrono
 

libcxx/modules/std/ranges.inc

@@ -138,9 +138,6 @@ export namespace std {
     }
 #endif // _LIBCPP_HAS_NO_LOCALIZATION
 
-#if _LIBCPP_STD_VER >= 23
-    // [range.adaptor.object], range adaptor objects
-    using std::ranges::range_adaptor_closure;
     // Note: This declaration not in the synopsis or explicitly in the wording.
     // However it is needed for the range adaptors.
     // [range.adaptor.object]/3
@@ -151,7 +148,11 @@ export namespace std {
     //   involving an object of type cv D as an operand to the | operator is
     //   undefined if overload resolution selects a program-defined operator|
     //   function.
+    // This is used internally in C++20 mode.
     using std::ranges::operator|;
+#if _LIBCPP_STD_VER >= 23
+    // [range.adaptor.object], range adaptor objects
+    using std::ranges::range_adaptor_closure;
 #endif
 
     // [range.all], all view

libcxx/test/CMakeLists.txt

@@ -1,5 +1,11 @@
 include(HandleLitArguments)
 add_subdirectory(tools)
+# When the tools add clang-tidy support, the dependencies need to be updated.
+# This cannot be done in the tools CMakeLists.txt since that does not update
+# the status in this (a parent) directory.
+if(TARGET cxx-tidy)
+  list(APPEND LIBCXX_TEST_DEPS cxx-tidy)
+endif()
 
 # By default, libcxx and libcxxabi share a library directory.
 if (NOT LIBCXX_CXX_ABI_LIBRARY_PATH)

libcxx/test/libcxx/clang_tidy.gen.py

@@ -26,6 +26,9 @@
 // The GCC compiler flags are not always compatible with clang-tidy.
 // UNSUPPORTED: gcc
 
+// Clang 17 has false positives.
+// UNSUPPORTED: clang-17
+
 {lit_header_restrictions.get(header, '')}
 
 // TODO: run clang-tidy with modules enabled once they are supported

libcxx/test/tools/clang_tidy_checks/CMakeLists.txt

@@ -64,6 +64,28 @@ if(NOT HAS_CLANG_TIDY_HEADERS)
                  "clang-tidy headers are not present.")
   return()
 endif()
+
+# The clangTidy plugin uses C++20, so ensure that we support C++20 when using libstdc++.
+# This is required because some versions of libstdc++ used as a system library on build platforms
+# we support do not support C++20 yet.
+# Note it has not been tested whether version 11 works.
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/test.cpp" "
+#include <version>
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 12
+  # error The libstdc++ version is too old.
+#endif
+int main(){}
+")
+try_compile(HAS_NEWER_STANDARD_LIBRARY
+  "${CMAKE_CURRENT_BINARY_DIR}"
+  "${CMAKE_CURRENT_BINARY_DIR}/test.cpp"
+   LINK_LIBRARIES clangTidy)
+
+if(NOT HAS_NEWER_STANDARD_LIBRARY)
+  message(STATUS "Clang-tidy tests are disabled due to using "
+                 "stdlibc++ older than version 12")
+  return()
+endif()
 message(STATUS "Clang-tidy tests are enabled.")
 
 set(SOURCES
@@ -88,5 +110,3 @@ set_target_properties(cxx-tidy PROPERTIES
 
 set_target_properties(cxx-tidy PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 set(CMAKE_SHARED_MODULE_SUFFIX_CXX .plugin) # Use a portable suffix to simplify how we can find it from Lit
-
-list(APPEND LIBCXX_TEST_DEPS cxx-tidy)