[llvm] d4abdd2 - [CMake] Check CMAKE_CXX_STANDARD and error if it's to old

[Tobias Hieta via llvm-commits]

Author: Tobias Hieta
Date: 2022-08-08T15:07:23+02:00
New Revision: d4abdd2e3dda8f94219879501d22361e0549592b

URL: https://github.com/llvm/llvm-project/commit/d4abdd2e3dda8f94219879501d22361e0549592b
DIFF: https://github.com/llvm/llvm-project/commit/d4abdd2e3dda8f94219879501d22361e0549592b.diff

LOG: [CMake] Check CMAKE_CXX_STANDARD and error if it's to old

Also nuke the old value out of the cache if it's there, otherwise
it could lead to problems when the project increases the standard
and the developer just runs the "make/ninja" program.

Reviewed By: royjacobson

Differential Revision: https://reviews.llvm.org/D131367

Added: 
    

Modified: 
    llvm/CMakeLists.txt

Removed: 
    


################################################################################
diff  --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index b0c6a15ef0e8..9d83eb503e27 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -58,8 +58,25 @@ project(LLVM
 # 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.