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
@@ -81,6 +81,8 @@
       endforeach()
     endif()
   endif()
+  # Enable assertions in libstdc++.
+  add_compile_definitions($<$<COMPILE_LANGUAGE:CXX>:_GLIBCXX_ASSERTIONS>)
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)