From 0eb8f018a1c8ba1bb22fcc1d070a7a93cda6df1a Mon Sep 17 00:00:00 2001
From: Alexey Samsonov <vonosmas@gmail.com>
Date: Wed, 25 Mar 2026 10:51:31 -0700
Subject: [PATCH] [libc++] Add CMake plumbing for _LIBCPP_HAS_THREAD_API_C11
 mode. (#188414)

Introduce CMake option `LIBCXX_HAS_C11_THREAD_API` that explicitly
instructs libc++ to use C11 threads. Make sure
`_LIBCPP_HAS_THREAD_API_C11` is defined properly in the
`<__config_site>` and interacts correctly with platform-based
auto-selection of thread API when it's not explicitly set.
---
 libcxx/CMakeLists.txt           | 24 ++++++++++++++++++++++++
 libcxx/include/__config         |  7 ++++++-
 libcxx/include/__config_site.in |  2 +-
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index e1620f91a453..f54c20370c18 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -310,6 +310,7 @@ option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
   "Build libc++ with an externalized threading API.
    This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON." OFF)
+option(LIBCXX_HAS_C11_THREAD_API "Ignore auto-detection and force use of C11 thread API" OFF)
 
 if (LIBCXX_ENABLE_THREADS)
   set(LIBCXX_PSTL_BACKEND "std_thread" CACHE STRING "Which PSTL backend to use")
@@ -354,6 +355,10 @@ if(NOT LIBCXX_ENABLE_THREADS)
     message(FATAL_ERROR "LIBCXX_HAS_WIN32_THREAD_API can only be set to ON"
                         " when LIBCXX_ENABLE_THREADS is also set to ON.")
   endif()
+  if (LIBCXX_HAS_C11_THREAD_API)
+    message(FATAL_ERROR "LIBCXX_HAS_C11_THREAD_API can only be set to ON"
+                        " when LIBCXX_ENABLE_THREADS is also set to ON.")
+  endif()
 
 endif()
 
@@ -368,6 +373,11 @@ if (LIBCXX_HAS_EXTERNAL_THREAD_API)
                         " and LIBCXX_HAS_WIN32_THREAD_API cannot be both"
                         " set to ON at the same time.")
   endif()
+  if (LIBCXX_HAS_C11_THREAD_API)
+    message(FATAL_ERROR "The options LIBCXX_HAS_EXTERNAL_THREAD_API"
+                        " and LIBCXX_HAS_C11_THREAD_API cannot be both"
+                        " set to ON at the same time.")
+  endif()
 endif()
 
 if (LIBCXX_HAS_PTHREAD_API)
@@ -376,6 +386,19 @@ if (LIBCXX_HAS_PTHREAD_API)
                         " and LIBCXX_HAS_WIN32_THREAD_API cannot be both"
                         " set to ON at the same time.")
   endif()
+  if (LIBCXX_HAS_C11_THREAD_API)
+    message(FATAL_ERROR "The options LIBCXX_HAS_PTHREAD_API"
+                        " and LIBCXX_HAS_C11_THREAD_API cannot be both"
+                        " set to ON at the same time.")
+  endif()
+endif()
+
+if (LIBCXX_HAS_WIN32_THREAD_API)
+  if (LIBCXX_HAS_C11_THREAD_API)
+    message(FATAL_ERROR "The options LIBCXX_HAS_WIN32_THREAD_API"
+                        " and LIBCXX_HAS_C11_THREAD_API cannot be both"
+                        " set to ON at the same time.")
+  endif()
 endif()
 
 if (NOT LIBCXX_ENABLE_RTTI AND LIBCXX_ENABLE_EXCEPTIONS)
@@ -762,6 +785,7 @@ endif()
 config_define(${LIBCXX_HAS_PTHREAD_API} _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define(${LIBCXX_HAS_EXTERNAL_THREAD_API} _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define(${LIBCXX_HAS_WIN32_THREAD_API} _LIBCPP_HAS_THREAD_API_WIN32)
+config_define(${LIBCXX_HAS_C11_THREAD_API} _LIBCPP_HAS_THREAD_API_C11)
 config_define(${LIBCXX_HAS_MUSL_LIBC} _LIBCPP_HAS_MUSL_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define(${LIBCXX_ENABLE_FILESYSTEM} _LIBCPP_HAS_FILESYSTEM)
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 9cb98bbb5934..942b0a9dbefa 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -370,7 +370,8 @@ typedef __char32_t char32_t;
 #  if _LIBCPP_HAS_THREADS &&                                                                                           \
       !_LIBCPP_HAS_THREAD_API_PTHREAD &&                                                                               \
       !_LIBCPP_HAS_THREAD_API_WIN32 &&                                                                                 \
-      !_LIBCPP_HAS_THREAD_API_EXTERNAL
+      !_LIBCPP_HAS_THREAD_API_EXTERNAL &&                                                                              \
+      !_LIBCPP_HAS_THREAD_API_C11
 
 #    if defined(__FreeBSD__) ||                                                                                        \
         defined(__wasi__) ||                                                                                           \
@@ -414,6 +415,10 @@ typedef __char32_t char32_t;
 #    error _LIBCPP_HAS_THREAD_API_EXTERNAL may only be true when _LIBCPP_HAS_THREADS is true.
 #  endif
 
+#  if !_LIBCPP_HAS_THREADS && _LIBCPP_HAS_THREAD_API_C11
+#    error _LIBCPP_HAS_THREAD_API_C11 may only be true when _LIBCPP_HAS_THREADS is true.
+#  endif
+
 #  if !_LIBCPP_HAS_MONOTONIC_CLOCK && _LIBCPP_HAS_THREADS
 #    error _LIBCPP_HAS_MONOTONIC_CLOCK may only be false when _LIBCPP_HAS_THREADS is false.
 #  endif
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
index eec6cb720ba5..e6e33a2eeb2a 100644
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -19,7 +19,7 @@
 #cmakedefine01 _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine01 _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine01 _LIBCPP_HAS_THREAD_API_WIN32
-#define _LIBCPP_HAS_THREAD_API_C11 0 // FIXME: Is this guarding dead code?
+#cmakedefine01 _LIBCPP_HAS_THREAD_API_C11
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine01 _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_NO_VCRUNTIME