From 1fc075df881f1b4c58f7e85b53aecb14fcba1e24 Mon Sep 17 00:00:00 2001
From: Thomas Madlener <thomas.madlener@desy.de>
Date: Mon, 8 Jul 2024 14:34:51 +0200
Subject: [PATCH] Detect more c++ standards and make sure useful message
 actually appears (#635)

* Detect more c++ standards and make sure useful message actually appears

* Order c++ standards numerically

* Use a for loop to detect ROOT c++ standard

* Add a bit more logging
---
 CMakeLists.txt | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index efcffb9f5..dfd6f98db 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -88,14 +88,20 @@ endif()
 # ROOT_CXX_STANDARD was introduced in https://github.com/root-project/root/pull/6466
 # before that it's an empty variable so we check if it's any number > 0
 if(NOT DEFINED ROOT_CXX_STANDARD)
+  message(STATUS "ROOT c++ standard not yet available. Determining from compile features")
   get_target_property(ROOT_COMPILE_FEATURES ROOT::Core INTERFACE_COMPILE_FEATURES)
-  if("cxx_std_17" IN_LIST ROOT_COMPILE_FEATURES)
-    set(ROOT_CXX_STANDARD 17)
-  elseif("cxx_std_20" IN_LIST ROOT_COMPILE_FEATURES)
-    set(ROOT_CXX_STANDARD 20)
-  else()
-    message(FATAL_ERROR "ROOT C++ could not be detected")
-  endif()
+  foreach(cxxstd IN ITEMS 20;17;14;11)
+    if ("cxx_std_${cxxstd}" IN_LIST ROOT_COMPILE_FEATURES)
+      set(ROOT_CXX_STANDARD ${cxxstd})
+      break()
+    endif()
+  endforeach()
+endif()
+
+if(NOT DEFINED ROOT_CXX_STANDARD)
+  message(WARNING "ROOT c++ standard could not be detected")
+else()
+  message(STATUS "Determined ROOT c++ standard: " ${ROOT_CXX_STANDARD})
 endif()
 
 if(ROOT_CXX_STANDARD VERSION_LESS 17)