diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -58,8 +58,25 @@
 # Must go after project(..)
 include(GNUInstallDirs)
 
-set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
+# This C++ standard is required to build LLVM.
+set(LLVM_REQUIRED_CXX_STANDARD 17)
+
+# If we find that the cache contains CMAKE_CXX_STANDARD it means that it's a old CMakeCache.txt
+# and we can just inform the user and then reset it.
+if($CACHE{CMAKE_CXX_STANDARD} AND $CACHE{CMAKE_CXX_STANDARD} LESS ${LLVM_REQUIRED_CXX_STANDARD})
+  message(WARNING "Resetting cache value for CMAKE_CXX_STANDARD to ${LLVM_REQUIRED_CXX_STANDARD}")
+  unset(CMAKE_CXX_STANDARD CACHE)
+endif()
+
+# if CMAKE_CXX_STANDARD is still set after the cache unset above it means that the user requested it
+# and we allow it to be set to something newer than the required standard but otherwise we fail.
+if(DEFINED CMAKE_CXX_STANDARD AND CMAKE_CXX_STANDARD LESS ${LLVM_REQUIRED_CXX_STANDARD})
+  message(FATAL_ERROR "Requested CMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD} which is less than the required ${LLVM_REQUIRED_CXX_STANDARD}.")
+endif()
+
+set(CMAKE_CXX_STANDARD ${LLVM_REQUIRED_CXX_STANDARD} CACHE STRING "C++ standard to conform to")
 set(CMAKE_CXX_STANDARD_REQUIRED YES)
+
 if (CYGWIN)
   # Cygwin is a bit stricter and lack things like 'strdup', 'stricmp', etc in
   # c++xx mode.