diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -83,6 +83,8 @@
   endif()
   # Enable assertions in libstdc++.
   add_compile_definitions(_GLIBCXX_ASSERTIONS)
+  # Enable assertions in libc++.
+  add_compile_definitions(_LIBCPP_ENABLE_ASSERTIONS)
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)