diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -64,8 +64,15 @@
 endif()
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
-  message(STATUS "No build type selected, default to Debug")
-  set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
+  message(FATAL_ERROR "
+No build type selected. You need to pass -DCMAKE_BUILD_TYPE=<type> in order to configure LLVM.
+Available options are:
+  * -DCMAKE_BUILD_TYPE=Release - For an optimized build with no assertions or debug info.
+  * -DCMAKE_BUILD_TYPE=Debug - For an unoptimized build with assertions and debug info.
+  * -DCMAKE_BUILD_TYPE=RelWithDebInfo - For an optimized build with no assertions but with debug info.
+  * -DCMAKE_BUILD_TYPE=MinSizeRel - For a build optimized for size instead of speed.
+Learn more about these options in our documentation at https://llvm.org/docs/CMake.html#cmake-build-type
+")
 endif()
 
 # Side-by-side subprojects layout: automatically set the
@@ -1252,11 +1259,11 @@
   set(BENCHMARK_ENABLE_INSTALL OFF CACHE BOOL "Don't install benchmark" FORCE)
   set(BENCHMARK_DOWNLOAD_DEPENDENCIES OFF CACHE BOOL "Don't download dependencies" FORCE)
   set(BENCHMARK_ENABLE_GTEST_TESTS OFF CACHE BOOL "Disable Google Test in benchmark" FORCE)
-  set(BENCHMARK_ENABLE_WERROR ${LLVM_ENABLE_WERROR} CACHE BOOL 
+  set(BENCHMARK_ENABLE_WERROR ${LLVM_ENABLE_WERROR} CACHE BOOL
     "Handle -Werror for Google Benchmark based on LLVM_ENABLE_WERROR" FORCE)
   # Since LLVM requires C++11 it is safe to assume that std::regex is available.
   set(HAVE_STD_REGEX ON CACHE BOOL "OK" FORCE)
-  add_subdirectory(${LLVM_THIRD_PARTY_DIR}/benchmark 
+  add_subdirectory(${LLVM_THIRD_PARTY_DIR}/benchmark
     ${CMAKE_CURRENT_BINARY_DIR}/third-party/benchmark)
   add_subdirectory(benchmarks)
 endif()
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -69,6 +69,14 @@
 Changes to building LLVM
 ------------------------
 
+* Omitting ``CMAKE_BUILD_TYPE`` when using a single configuration generator is now
+  an error. You now have to pass ``-DCMAKE_BUILD_TYPE=<type>`` in order to configure
+  LLVM. This is done to help new users of LLVM select the correct type: since building
+  LLVM in Debug mode is very resource intensive, we want to make sure that new users
+  make the choice that lines up with their usage. We have also improved documentation
+  around this setting that should help new users. You can find this documentation
+  `here <https://llvm.org/docs/CMake.html#cmake-build-type>`_.
+
 Changes to TableGen
 -------------------