MSVC: Add flags for C++ language standards

Visual Studio 2015 Update 3 introduced the notion of language standard
levels to MSVC.  The language standard level is defined in `_MSVC_LANG`
instead of `__cplusplus`.  It also added support for the `-std:c++14`
and `-std:c++latest` flags, although the compiler defaults to its C++14
mode anyway.  Visual Studio 2017 Update 3 will introduce support for the
`-std:c++17` flag.

Fixes: #16482

Modules/CMakeCXXCompilerId.cpp.in

@@ -27,12 +27,18 @@ char const *info_cray = "INFO" ":" "compiler_wrapper[CrayPrgEnv]";
 @CMAKE_CXX_COMPILER_ID_PLATFORM_CONTENT@
 @CMAKE_CXX_COMPILER_ID_ERROR_FOR_TEST@
 
+#if defined(_MSC_VER) && defined(_MSVC_LANG)
+#define CXX_STD _MSVC_LANG
+#else
+#define CXX_STD __cplusplus
+#endif
+
 const char* info_language_dialect_default = "INFO" ":" "dialect_default["
-#if __cplusplus > 201402L
+#if CXX_STD > 201402L
   "17"
-#elif __cplusplus >= 201402L
+#elif CXX_STD >= 201402L
   "14"
-#elif __cplusplus >= 201103L
+#elif CXX_STD >= 201103L
   "11"
 #else
   "98"

Modules/Compiler/MSVC-CXX.cmake

@@ -3,7 +3,25 @@
 
 include(Compiler/CMakeCommonCompilerMacros)
 
-if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
+if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.10.25017)
+  # VS 2015 Update 3 and above support language standard level flags,
+  # with the default and minimum level being C++14.
+  set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std:c++14")
+  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std:c++14")
+  if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.11.25505)
+    set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std:c++17")
+    set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std:c++17")
+  else()
+    set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std:c++latest")
+    set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std:c++latest")
+  endif()
+
+  __compiler_check_default_language_standard(CXX 19.0 14)
+elseif (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
   # MSVC has no specific options to set language standards, but set them as
   # empty strings anyways so the feature test infrastructure can at least check
   # to see if they are defined.

Tests/CompileFeatures/default_dialect.cpp

@@ -2,25 +2,30 @@
 template <long l>
 struct Outputter;
 
+#if defined(_MSC_VER) && defined(_MSVC_LANG)
+#define CXX_STD _MSVC_LANG
+#else
+#define CXX_STD __cplusplus
+#endif
+
 #if DEFAULT_CXX17
-#if __cplusplus <= 201402L
-Outputter<__cplusplus> o;
+#if CXX_STD <= 201402L
+Outputter<CXX_STD> o;
 #endif
 #elif DEFAULT_CXX14
-#if __cplusplus != 201402L
-Outputter<__cplusplus> o;
+#if CXX_STD != 201402L
+Outputter<CXX_STD> o;
 #endif
 #elif DEFAULT_CXX11
-#if __cplusplus != 201103L
-Outputter<__cplusplus> o;
+#if CXX_STD != 201103L
+Outputter<CXX_STD> o;
 #endif
 #else
 #if !DEFAULT_CXX98
 #error Buildsystem error
 #endif
-#if __cplusplus != 199711L && __cplusplus != 1 &&                             \
-  !defined(__GXX_EXPERIMENTAL_CXX0X__)
-Outputter<__cplusplus> o;
+#if CXX_STD != 199711L && CXX_STD != 1 && !defined(__GXX_EXPERIMENTAL_CXX0X__)
+Outputter<CXX_STD> o;
 #endif
 #endif