Check for supported compilers. (#2411)

Utter a dire warning for those who try to build with outdated and/or weird compilers.
XLC is no longer a thing, so we remove checking for it.

CMakeLists.txt

@@ -201,13 +201,10 @@ set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS "YES")
 
-# Detect and deprecate xlC.
-
-include("CheckCompilerXLC")
-
 # Compiler options common to library, examples, tests, etc.
 
 include("CompilerOptions")
+check_supported_cxx()
 add_compile_options("$<$<COMPILE_LANGUAGE:CXX>:${CXXOPT_WALL}>")
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CUDA_STANDARD 20)

cmake/CompilerOptions.cmake

@@ -74,24 +74,28 @@ set(CXXOPT_WALL
     #   because there is nothing to fix on our side.
 
     $<IF:$<CXX_COMPILER_ID:GNU>,-Wno-psabi,>
-
-    # Intel:
-    #
-    # Disable warning for unused template parameter
-    # this is raised by a templated function in the json library.
-
-    $<IF:$<CXX_COMPILER_ID:Intel>,-wd488,>)
+)
+
+# Check for supported compilers / versions
+function(check_supported_cxx)
+    set(cxx_supported_ids "AppleClang" "GNU" "Clang")
+    set(cxx_supported_ver           15    12      12)
+    foreach(id ver IN ZIP_LISTS cxx_supported_ids cxx_supported_ver)
+         if(CMAKE_CXX_COMPILER_ID MATCHES ${id} AND CMAKE_CXX_COMPILER_VERSION GREATER_EQUAL ${ver})
+             return()
+         endif()
+    endforeach()
+    message(WARNING "Found an unsupported compiler ${CMAKE_CXX_COMPILER_ID} version ${CMAKE_CXX_COMPILER_VERSION}, please consider switching to a supported version of GCC or Clang. Build failure is expected. We reserve the option to close all related issues without consideration.")
+endfunction()
 
 
 # Set ${optvar} in parent scope according to requested architecture.
 # Architectures are given by the same names that GCC uses for its
 # -mcpu or -march options.
-
 function(set_arch_target optvar optvar_cuda_guarded arch)
     if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
         # Correct compiler option unfortunately depends upon the target architecture family.
         # Extract this information from running the configured compiler with --verbose.
-
         try_compile(ignore ${CMAKE_BINARY_DIR} ${PROJECT_SOURCE_DIR}/cmake/dummy.cpp COMPILE_DEFINITIONS --verbose OUTPUT_VARIABLE cc_out)
         string(REPLACE "\n" ";" cc_out "${cc_out}")
         set(target)
@@ -106,22 +110,22 @@ function(set_arch_target optvar optvar_cuda_guarded arch)
         # architecture.
         # See clang / gcc manuals and:
         # https://maskray.me/blog/2022-08-28-march-mcpu-mtune
-        if (CMAKE_CXX_COMPILER_ID MATCHES "AppleClang" AND CMAKE_CXX_COMPILER_VERSION LESS 15)
-            set(arch_opt "")
+        if(CMAKE_CXX_COMPILER_ID MATCHES "AppleClang")
+            set(arch_opt "-march=${arch}")
         elseif (CMAKE_CXX_COMPILER_ID MATCHES "GNU")
-            if ("${target}" MATCHES "(arm64|aarch64)-.*")
+            if("${target}" MATCHES "(arm64|aarch64)-.*")
                 # on AArch64, this is correct, ...
                 set(arch_opt "-mcpu=${arch} -mtune=${arch}")
             else()
                 # ... however on x86 mcpu _is_ mtune _and_ deprecated (since 2003!), but ...
                 set(arch_opt "-march=${arch}")
-            endif ()
+            endif()
         elseif (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
             # ... clang likes march (and possibly mtune)
             # See https://discourse.llvm.org/t/when-to-use-mcpu-versus-march/47953/9
             set(arch_opt "-march=${arch} -mtune=${arch}")
-        else ()
-            message(STATUS "Falling back to -march=${arch} for compiler ${CMAKE_CXX_COMPILER_ID}")
+        else()
+            message(STATUS "Setting fallback architecture flags for ${CMAKE_CXX_COMPILER}.")
             set(arch_opt "-march=${arch}")
         endif()
     endif()
@@ -140,7 +144,6 @@ function(set_arch_target optvar optvar_cuda_guarded arch)
     else()
         set("${optvar_cuda_guarded}" "${arch_opt}" PARENT_SCOPE)
     endif()
-
 endfunction()
 
 # Set ${has_sve} and ${sve_length} in parent scope according to auto detection.