[libc++] Fix libc++ build with assertions enabled

[ldionne]

Summary

This fixes http://llvm.org/PR50534. This is another take on D103960
which is less disruptive.

libcxx/cmake/caches/Generic-assertions.cmake

+set(LIBCXX_ENABLE_ASSERTIONS ON CACHE BOOL "")

libcxx/include/__config

 #elif _LIBCPP_DEBUG == 0
 # define _LIBCPP_DEBUG_LEVEL 1
 #elif _LIBCPP_DEBUG == 1
 # define _LIBCPP_DEBUG_LEVEL 2
 #else
 # error Supported values for _LIBCPP_DEBUG are 0 and 1
 #endif
 
-// _LIBCPP_DEBUG_LEVEL is always defined to one of [0, 1, 2] at this point
-#if _LIBCPP_DEBUG_LEVEL >= 1 && !defined(_LIBCPP_DISABLE_EXTERN_TEMPLATE)
-# define _LIBCPP_EXTERN_TEMPLATE(...)
-#endif
-
-#ifdef _LIBCPP_DISABLE_EXTERN_TEMPLATE
-# define _LIBCPP_EXTERN_TEMPLATE(...)
-# define _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE(...)
-#endif
-
-#ifndef _LIBCPP_EXTERN_TEMPLATE
-#define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
-#endif
-
-// When the Debug mode is enabled, we disable extern declarations because we
-// don't want to use the functions compiled in the library, which might not
-// have had the debug mode enabled when built. However, some extern declarations
-// need to be used, because code correctness depends on it (several instances
-// in the <locale>). Those special declarations are declared with
-// _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE, which is enabled even
-// when the debug mode is enabled.
-#ifndef _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE
-# define _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE(...) extern template __VA_ARGS__;
+// Libc++ allows disabling extern template instantiation declarations by
+// means of users defining _LIBCPP_DISABLE_EXTERN_TEMPLATE.
+//
+// Furthermore, when the Debug mode is enabled, we disable extern declarations
+// when building user code because we don't want to use the functions compiled
+// in the library, which might not have had the debug mode enabled when built.
+// However, some extern declarations need to be used, because code correctness
+// depends on it (several instances in <locale>). Those special declarations
+// are declared with _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE, which is enabled
+// even when the debug mode is enabled.
+#if defined(_LIBCPP_DISABLE_EXTERN_TEMPLATE)
+#   define _LIBCPP_EXTERN_TEMPLATE(...) /* nothing */
+#   define _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE(...) /* nothing */
+#elif _LIBCPP_DEBUG_LEVEL >= 1 && !defined(_LIBCPP_BUILDING_LIBRARY)
+#   define _LIBCPP_EXTERN_TEMPLATE(...) /* nothing */
+#   define _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE(...) extern template __VA_ARGS__;
+#else
+#   define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
+#   define _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE(...) extern template __VA_ARGS__;
 #endif
 
-#define _LIBCPP_EXTERN_TEMPLATE_DEFINE(...) template __VA_ARGS__;
-
 #if defined(__APPLE__) || defined(__FreeBSD__) || defined(_LIBCPP_MSVCRT_LIKE) || \
     defined(__sun__) || defined(__NetBSD__) || defined(__CloudABI__)
 #define _LIBCPP_LOCALE__L_EXTENSIONS 1
 #endif
 
 #ifdef __FreeBSD__
 #define _DECLARE_C99_LDBL_MATH 1
 #endif

libcxx/src/string.cpp

 #include "stdexcept"
 #include <stdio.h>
 #include "__debug"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS __basic_string_common<true>;
 
+#define _LIBCPP_EXTERN_TEMPLATE_DEFINE(...) template __VA_ARGS__;
 #ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 #else
 _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 #endif
+#undef _LIBCPP_EXTERN_TEMPLATE_DEFINE
 
-template
-    string
-    operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);
+template string operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);
 
 namespace
 {
 
 template<typename T>
 inline
 void throw_helper( const string& msg )
 {

libcxx/utils/ci/buildkite-pipeline.yml

artifact_paths:
       - "**/test-results.xml"
     agents:
       queue: "libcxx-builders"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
 
+  - label: "Assertions enabled"
+    command: "libcxx/utils/ci/run-buildbot generic-assertions"
+    artifact_paths:
+      - "**/test-results.xml"
+      - "**/*.abilist"
+    agents:
+      queue: "libcxx-builders"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+
   - label: "Debug iterators"
     command: "libcxx/utils/ci/run-buildbot generic-debug-iterators"
     artifact_paths:
       - "**/test-results.xml"
       - "**/*.abilist"
     agents:
       queue: "libcxx-builders"
     retry:

libcxx/utils/ci/run-buildbot

generic-cxx2b)
     export CC=clang-tot
     export CXX=clang++-tot
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-cxx2b.cmake" \
                    -DLIBCXX_TEST_CONFIG="${MONOREPO_ROOT}/libcxx/test/configs/libcxx-trunk-shared.cfg.in"
     check-cxx-cxxabi
     check-abi-list
 ;;
+generic-assertions)
+    export CC=clang
+    export CXX=clang++
+    clean
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-assertions.cmake"
+    check-cxx-cxxabi
+    check-abi-list
+;;
 generic-debug-iterators)
     export CC=clang-tot
     export CXX=clang++-tot
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-debug-iterators.cmake"
     check-cxx-cxxabi
     check-abi-list
 ;;