Merge pull request kokkos#230 from mhoemmen/Fix-CMake-CXX-version

Permit C++ < 20

CMakeLists.txt

@@ -18,42 +18,72 @@ option(LINALG_ENABLE_EXAMPLES "Build examples." Off)
 
 # Option to override which C++ standard to use
 set(LINALG_CXX_STANDARD DETECT CACHE STRING "Override the default CXX_STANDARD to compile with.")
-set_property(CACHE LINALG_CXX_STANDARD PROPERTY STRINGS DETECT 20)
+set_property(CACHE LINALG_CXX_STANDARD PROPERTY STRINGS DETECT 14 17 20 23)
 
 option(LINALG_ENABLE_CONCEPTS "Try to enable concepts support by giving extra flags." On)
 
 ################################################################################
 
-# Make sure that the compiler supports C++20
-if(LINALG_CXX_STANDARD STREQUAL "20")
+# Decide on the standard to use
+if(LINALG_CXX_STANDARD STREQUAL "17")
+  if("cxx_std_17" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    message(STATUS "Using C++17 standard")
+    set(CMAKE_CXX_STANDARD 17)
+  else()
+    message(FATAL_ERROR "Requested LINALG_CXX_STANDARD \"17\" not supported by provided C++ compiler")
+  endif()
+elseif(LINALG_CXX_STANDARD STREQUAL "14")
+  if("cxx_std_14" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    message(STATUS "Using C++14 standard")
+    set(CMAKE_CXX_STANDARD 14)
+  else()
+    message(FATAL_ERROR "Requested LINALG_CXX_STANDARD \"14\" not supported by provided C++ compiler")
+  endif()
+elseif(LINALG_CXX_STANDARD STREQUAL "20")
   if("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
     message(STATUS "Using C++20 standard")
     set(CMAKE_CXX_STANDARD 20)
   else()
     message(FATAL_ERROR "Requested LINALG_CXX_STANDARD \"20\" not supported by provided C++ compiler")
   endif()
+elseif(LINALG_CXX_STANDARD STREQUAL "23")
+  if("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    message(STATUS "Using C++23 standard")
+    set(CMAKE_CXX_STANDARD 23)
+  else()
+    message(FATAL_ERROR "Requested LINALG_CXX_STANDARD \"23\" not supported by provided C++ compiler")
+  endif()
 else()
-  if("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+  if("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    set(CMAKE_CXX_STANDARD 23)
+    message(STATUS "Detected support for C++23 standard")
+  elseif("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
     set(CMAKE_CXX_STANDARD 20)
     message(STATUS "Detected support for C++20 standard")
+  elseif("cxx_std_17" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    set(CMAKE_CXX_STANDARD 17)
+    message(STATUS "Detected support for C++17 standard")
+  elseif("cxx_std_14" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    set(CMAKE_CXX_STANDARD 14)
+    message(STATUS "Detected support for C++14 standard")
   else()
-    message(FATAL_ERROR "Cannot detect CXX_STANDARD of C++20")
+    message(FATAL_ERROR "Cannot detect CXX_STANDARD of C++14 or newer.")
   endif()
 endif()
 
 ################################################################################
 
 if(LINALG_ENABLE_CONCEPTS)
-  if(CMAKE_CXX_STANDARD STREQUAL "20")
+  if(CMAKE_CXX_STANDARD GREATER_EQUAL 20)
     include(CheckCXXCompilerFlag)
-    CHECK_CXX_COMPILER_FLAG("-fconcepts" COMPILER_SUPPORTS_FCONCEPTS)
+    check_cxx_compiler_flag("-fconcepts" COMPILER_SUPPORTS_FCONCEPTS)
     if(COMPILER_SUPPORTS_FCONCEPTS)
-      message(STATUS "-- Using \"-fconcepts\" to enable concepts support")
+      message(STATUS "Using \"-fconcepts\" to enable concepts support")
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fconcepts")
     else()
-      CHECK_CXX_COMPILER_FLAG("-fconcepts-ts" COMPILER_SUPPORTS_FCONCEPTS_TS)
+      check_cxx_compiler_flag("-fconcepts-ts" COMPILER_SUPPORTS_FCONCEPTS_TS)
       if(COMPILER_SUPPORTS_FCONCEPTS)
-        message(STATUS "-- Using \"-fconcepts-ts\" to enable concepts support")
+        message(STATUS "Using \"-fconcepts-ts\" to enable concepts support")
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fconcepts-ts")
       endif()
     endif()

README.md

@@ -2,21 +2,20 @@
 
 This is a reference implementation of P1673,
 "A free function linear algebra interface based on the BLAS."
-You can find the latest revision of P1673 at wg21.link/p1673.
+You can find the latest submitted revision of P1673
+[at this URL](https://wg21.link/p1673).
 
 ## Requirements
 
   - CMake >= 3.17 (earlier versions may work, but are not tested)
-  - C++ build environment that supports C++20
+  - C++ build environment that supports C++17 or greater
 
 ## Tested compilers
 
 We run github's automated tests on every pull request.
 Automated tests use "ubuntu-latest",
 which presumably defaults to a fairly new GCC.
-
-Before we started requiring concepts, MSVC 2019 16.7.0 was able to build and run tests and examples as of 2021/05/27.
-We will start testing on MSVC 2019 16.10 soon, which claims full support for C++20.
+Other compilers, including MSVC 2019, have been tested in the past.
 
 ## Brief build instructions