Drop support for C++11 standard

Author: Growl1234

CMakeLists.txt

@@ -227,8 +227,11 @@ set(ABACUS_BIN_PATH ${CMAKE_CURRENT_BINARY_DIR}/${ABACUS_BIN_NAME})
 include_directories(${ABACUS_SOURCE_DIR})
 include_directories(${ABACUS_SOURCE_DIR}/source_base/module_container)
 
+set(ABACUS_MIN_CXX_STANDARD 14)
 if(NOT DEFINED CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD ${ABACUS_MIN_CXX_STANDARD})
+elseif(CMAKE_CXX_STANDARD LESS ABACUS_MIN_CXX_STANDARD)
+  message(FATAL_ERROR "ABACUS requires C++14 or newer.")
 endif()
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
@@ -331,7 +334,6 @@ if(ENABLE_LCAO)
     target_link_libraries(${ABACUS_BIN_NAME} ${PEXSI_LIBRARY} ${SuperLU_DIST_LIBRARY} ${ParMETIS_LIBRARY} ${METIS_LIBRARY} pexsi)
     include_directories(${PEXSI_INCLUDE_DIR} ${ParMETIS_INCLUDE_DIR})
     add_compile_definitions(__PEXSI)
-    set(CMAKE_CXX_STANDARD 14)
   endif()
 else()
   set(ENABLE_MLALGO OFF)
@@ -452,7 +454,6 @@ if(USE_CUDA)
   # Always find CUDAToolkit to get CUDAToolkit_VERSION
   find_package(CUDAToolkit REQUIRED)
 
-  set_if_higher(CMAKE_CXX_STANDARD 14)
   if(CUDAToolkit_VERSION VERSION_GREATER_EQUAL "13.0")
     message(STATUS "CUDA ${CUDAToolkit_VERSION} detected. Setting CMAKE_CUDA_STANDARD to 17.")
     set_if_higher(CMAKE_CXX_STANDARD 17)
@@ -687,8 +688,6 @@ if(ENABLE_MLALGO OR DEFINED Torch_DIR)
   find_package(Torch REQUIRED)
   if(NOT Torch_VERSION VERSION_LESS "2.1.0")
     set_if_higher(CMAKE_CXX_STANDARD 17)
-  elseif(NOT Torch_VERSION VERSION_LESS "1.5.0")
-    set_if_higher(CMAKE_CXX_STANDARD 14)
   endif()
   include_directories(${TORCH_INCLUDE_DIRS})
   if(MKL_FOUND)
@@ -750,7 +749,6 @@ if(DEFINED LIBRI_DIR)
   set(ENABLE_LIBRI ON)
 endif()
 if(ENABLE_LIBRI)
-  set_if_higher(CMAKE_CXX_STANDARD 14)
   if(LIBRI_DIR)
   else()
     find_package(LibRI REQUIRED)