diff --git a/lldb/CMakeLists.txt b/lldb/CMakeLists.txt
--- a/lldb/CMakeLists.txt
+++ b/lldb/CMakeLists.txt
@@ -124,6 +124,11 @@
           message(WARNING "LLDB test suite requires libc++ in llvm/projects/libcxx or an existing build symlinked to ${cxx_dir}")
         endif()
       else()
+        if(NOT libcxx IN_LIST LLVM_ENABLE_PROJECTS)
+          message(FATAL_ERROR
+            "LLDB test suite requires libc++, but it is currently disabled. "
+            "Please add `libcxx` to `LLVM_ENABLE_PROJECTS`.")
+        endif()
         list(APPEND LLDB_TEST_DEPS cxx)
       endif()
     endif()
diff --git a/lldb/docs/resources/build.rst b/lldb/docs/resources/build.rst
--- a/lldb/docs/resources/build.rst
+++ b/lldb/docs/resources/build.rst
@@ -152,6 +152,12 @@
 LLVM, then you can pass LLVM-specific CMake variables to cmake when building
 LLDB.
 
+If you are building both Clang and LLDB together, be sure to also add libc++,
+which is currently required for testing on macOS:
+
+::
+
+  > cmake -D LLVM_ENABLE_PROJECTS='clang;lldb;libcxx' $PATH_TO_LLVM -G Ninja
 
 Here are some commonly used LLDB-specific CMake variables: