diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -116,20 +116,17 @@
 # LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS
 # This allows an easy way of setting up a build directory for llvm and another
 # one for llvm+clang+... using the same sources.
-set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly;pstl")
-# The flang project is not yet part of "all" projects (see C++ requirements)
-set(LLVM_EXTRA_PROJECTS "flang")
+set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;flang;libc;libclc;lld;lldb;mlir;openmp;polly;pstl")
 # List of all known projects in the mono repo
-set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
-    "Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
+    "Semicolon-separated list of projects to build (${LLVM_ALL_PROJECTS}), or \"all\".")
 # Make sure expansion happens first to not handle "all" in rest of the checks.
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()
 foreach(proj ${LLVM_ENABLE_PROJECTS})
-  if (NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
-     MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}. Did you mean to enable it as a runtime in LLVM_ENABLE_RUNTIMES?")
+  if (NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_ALL_PROJECTS)
+      MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_ALL_PROJECTS}. Did you mean to enable it as a runtime in LLVM_ENABLE_RUNTIMES?")
   endif()
 endforeach()
 
@@ -190,7 +187,7 @@
 
 if (LLVM_ENABLE_PROJECTS_USED OR NOT LLVM_ENABLE_PROJECTS STREQUAL "")
   set(LLVM_ENABLE_PROJECTS_USED ON CACHE BOOL "" FORCE)
-  foreach(proj ${LLVM_KNOWN_PROJECTS} ${LLVM_EXTERNAL_PROJECTS})
+  foreach(proj ${LLVM_ALL_PROJECTS} ${LLVM_EXTERNAL_PROJECTS})
     string(TOUPPER "${proj}" upper_proj)
     string(REGEX REPLACE "-" "_" upper_proj ${upper_proj})
     if ("${proj}" IN_LIST LLVM_ENABLE_PROJECTS)