diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -64,8 +64,16 @@
 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 optimized build with no assertions or debug info.
+  * -DCMAKE_BUILD_TYPE=Debug - For unoptimized build with assertions and debug info.
+  * -DCMAKE_BUILD_TYPE=RelWithDebInfo - For optimized build with no assertions but with debug info.
+  * -DCMAKE_BUILD_TYPE=MinSizeRel - Build optimized for size instead of speed.
+Learn more about these options and which value is right for your use case in our documentation:
+https://llvm.org/docs/CMake.html
+")
 endif()
 
 # Side-by-side subprojects layout: automatically set the
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -69,6 +69,10 @@
 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.
+
 Changes to TableGen
 -------------------