Implement CMake upgrade and C++ standard deprecation as we did with C++11

cmake/CMakeLists.txt

@@ -2,7 +2,7 @@
 ########################################
 # CMake build system
 # This file is part of LAMMPS
-cmake_minimum_required(VERSION 3.16)
+cmake_minimum_required(VERSION 3.20)
 ########################################
 # set policy to silence warnings about ignoring <PackageName>_ROOT but use it
 if(POLICY CMP0074)
@@ -144,16 +144,28 @@ if((PKG_KOKKOS) AND (Kokkos_ENABLE_CUDA) AND NOT (CMAKE_CXX_COMPILER_ID STREQUAL
   set(CMAKE_TUNE_DEFAULT "${CMAKE_TUNE_DEFAULT} -Xcudafe --diag_suppress=unrecognized_pragma")
 endif()
 
-# we require C++11 without extensions. Kokkos requires at least C++17 (currently)
+# we *require* C++11 without extensions but prefer C++17.
+# Kokkos requires at least C++17 (currently)
 if(NOT CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 11)
+  if(cxx_std_17 IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    set(CMAKE_CXX_STANDARD 17)
+  else()
+    set(CMAKE_CXX_STANDARD 11)
+  endif()
 endif()
 if(CMAKE_CXX_STANDARD LESS 11)
   message(FATAL_ERROR "C++ standard must be set to at least 11")
 endif()
+if(CMAKE_CXX_STANDARD LESS 17)
+  message(WARNING "Selecting C++17 standard is preferred over C++${CMAKE_CXX_STANDARD}")
+endif()
 if(PKG_KOKKOS AND (CMAKE_CXX_STANDARD LESS 17))
   set(CMAKE_CXX_STANDARD 17)
 endif()
+# turn off C++17 check in lmptype.h
+if(LAMMPS_CXX11)
+  add_compile_definitions(LAMMPS_CXX11)
+endif()
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF CACHE BOOL "Use compiler extensions")
 # ugly hacks for MSVC which by default always reports an old C++ standard in the __cplusplus macro

src/MAKE/Makefile.mpi

@@ -7,12 +7,12 @@ SHELL = /bin/sh
 # specify flags and libraries needed for your compiler
 
 CC =		mpicxx
-CCFLAGS =	-g -O3 -std=c++11
+CCFLAGS =	-g -O3 # -std=c++17
 SHFLAGS =	-fPIC
 DEPFLAGS =	-M
 
 LINK =		mpicxx
-LINKFLAGS =	-g -O3 -std=c++11
+LINKFLAGS =	-g -O3 # -std=c++17
 LIB =
 SIZE =		size
 
@@ -28,7 +28,7 @@ SHLIBFLAGS =	-shared -rdynamic
 # LAMMPS ifdef settings
 # see possible settings in Section 3.5 of the manual
 
-LMP_INC =	-DLAMMPS_GZIP -DLAMMPS_MEMALIGN=64  # -DLAMMPS_CXX98
+LMP_INC =	-DLAMMPS_GZIP -DLAMMPS_MEMALIGN=64  # -DLAMMPS_CXX11
 
 # MPI library
 # see discussion in Section 3.4 of the manual

src/MAKE/Makefile.serial

@@ -7,12 +7,12 @@ SHELL = /bin/sh
 # specify flags and libraries needed for your compiler
 
 CC =		g++
-CCFLAGS =	-g -O3 -std=c++11
+CCFLAGS =	-g -O3 # -std=c++17
 SHFLAGS =	-fPIC
 DEPFLAGS =	-M
 
 LINK =		g++
-LINKFLAGS =	-g -O -std=c++11
+LINKFLAGS =	-g -O # -std=c++17
 LIB =
 SIZE =		size
 
@@ -28,7 +28,7 @@ SHLIBFLAGS =	-shared -rdynamic
 # LAMMPS ifdef settings
 # see possible settings in Section 3.5 of the manual
 
-LMP_INC =	-DLAMMPS_GZIP -DLAMMPS_MEMALIGN=64  # -DLAMMPS_CXX98
+LMP_INC =	-DLAMMPS_GZIP -DLAMMPS_MEMALIGN=64  # -DLAMMPS_CXX11
 
 # MPI library
 # see discussion in Section 3.4 of the manual

src/lmptype.h

@@ -34,6 +34,13 @@
 #error LAMMPS requires a C++11 (or later) compliant compiler. Enable C++11 compatibility or upgrade the compiler.
 #endif
 
+// C++17 check
+#ifndef LAMMPS_CXX11
+#if __cplusplus < 201703L
+#error LAMMPS is planning to transition to C++17. To disable this error please use a C++17 compliant compiler, enable C++17 support, or define -DLAMMPS_CXX11 in your makefile or when running cmake
+#endif
+#endif
+
 #ifndef __STDC_LIMIT_MACROS
 #define __STDC_LIMIT_MACROS
 #endif