Clarification of C++ standard usage (#4839)

CMakeLists.txt

@@ -240,6 +240,9 @@ if(GPERF)
   endif()
 endif()
 
+# Despite setting C++ standard to 20, features from this version are not being used.
+# The oldest compiler used is GCC 6.5 - and that defines our C++ feature set (meaning most of C++17).
+# It's present only to take advantage of fmt::format build time errors.
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_CXX_STANDARD_REQUIRED OFF)

docs/CONTRIBUTING.md

@@ -3,6 +3,11 @@
 This guide outlines useful resources, tools and processes for contribution to
 DevilutionX.
 
+## C++ Standard
+Despite setting C++ standard to 20 in CMakeLists.txt, features from this version are not being used.
+The oldest compiler used is GCC 6.5 - and that defines our C++ feature set (meaning most of C++17).
+It's present only to take advantage of fmt::format build time errors.
+
 ## Code style guide
 
 [The code style guide](https://github.com/diasurgical/devilutionX/wiki/Code-Style) is evolving with the project.