build: remove LLVM_CXX_STD extension point

This extension point is not needed. Provide the equivalent option
through `CMAKE_CXX_STANDARD` which mirrors the previous extension point. Rely on
CMake to provide the check for the compiler instead.

Author: compnerd

llvm/CMakeLists.txt

@@ -50,6 +50,10 @@ project(LLVM
   VERSION ${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}
   LANGUAGES C CXX ASM)
 
+set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+set(CMAKE_CXX_STANDARD_REQUIRED YES)
+set(CMAKE_CXX_EXTENSIONS NO)
+
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")
   set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -18,17 +18,6 @@ else()
   set(LINKER_IS_LLD_LINK FALSE)
 endif()
 
-set(LLVM_CXX_STD_default "c++14")
-# Preserve behaviour of legacy cache variables
-if (LLVM_ENABLE_CXX1Z)
-  set(LLVM_CXX_STD_default "c++1z")
-endif()
-if (LLVM_CXX_STD STREQUAL "c++11")
-  set(LLVM_CXX_STD_force FORCE)
-endif()
-set(LLVM_CXX_STD ${LLVM_CXX_STD_default}
-    CACHE STRING "C++ standard to use for compilation." ${LLVM_CXX_STD_force})
-
 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
 string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 
@@ -445,23 +434,6 @@ if ( LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL" )
   add_flag_if_supported("-Werror=unguarded-availability-new" WERROR_UNGUARDED_AVAILABILITY_NEW)
 endif( LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL" )
 
-# C++ language standard selection for compilers accepting the GCC-style option:
-if ( LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL" )
-  check_cxx_compiler_flag("-std=${LLVM_CXX_STD}" CXX_SUPPORTS_CXX_STD)
-  if (CXX_SUPPORTS_CXX_STD)
-   if (CYGWIN OR MINGW)
-      # MinGW and Cygwin are a bit stricter and lack things like
-      # 'strdup', 'stricmp', etc in c++11 mode.
-      string(REPLACE "c++" "gnu++" gnu_LLVM_CXX_STD "${LLVM_CXX_STD}")
-      append("-std=${gnu_LLVM_CXX_STD}" CMAKE_CXX_FLAGS)
-    else()
-      append("-std=${LLVM_CXX_STD}" CMAKE_CXX_FLAGS)
-    endif()
-  else()
-    message(FATAL_ERROR "The host compiler does not support '-std=${LLVM_CXX_STD}'.")
-  endif()
-endif( LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL" )
-
 # Modules enablement for GCC-compatible compilers:
 if ( LLVM_COMPILER_IS_GCC_COMPATIBLE AND LLVM_ENABLE_MODULES )
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})

llvm/docs/CMake.rst

@@ -204,6 +204,17 @@ CMake manual, or execute ``cmake --help-variable VARIABLE_NAME``.
 **CMAKE_CXX_FLAGS**:STRING
   Extra flags to use when compiling C++ source files.
 
+Rarely-used CMake variables
+---------------------------
+
+Here are some of the CMake variables that are rarely used, along with a brief
+explanation and LLVM-specific notes.  For full documentation, consult the CMake
+manual, or execute ``cmake --help-variable VARIABLE_NAME``.
+
+**CMAKE_CXX_STANDARD**:STRING
+  Sets the C++ standard to conform to when building LLVM.  Possible values are
+  14, 17, 20.  LLVM Requires C++ 14 or higher.  This defaults to 14.
+
 .. _LLVM-specific variables:
 
 LLVM-specific variables
@@ -274,9 +285,6 @@ LLVM-specific variables
   Enable unwind tables in the binary.  Disabling unwind tables can reduce the
   size of the libraries.  Defaults to ON.
 
-**LLVM_CXX_STD**:STRING
-  Build with the specified C++ standard. Defaults to "c++11".
-
 **LLVM_ENABLE_ASSERTIONS**:BOOL
   Enables code assertions. Defaults to ON if and only if ``CMAKE_BUILD_TYPE``
   is *Debug*.