[clang-cl]: Add /std:c++23preview and update _MSVC_LANG for C++23 (llvm#112378)

As discussed in
https://discourse.llvm.org/t/clang-cl-adding-std-c-23preview/82553

Author: zmodem

clang/docs/ReleaseNotes.rst

@@ -605,6 +605,8 @@ Android Support
 Windows Support
 ^^^^^^^^^^^^^^^
 
+- clang-cl now supports ``/std:c++23preview`` which enables C++23 features.
+
 - Clang no longer allows references inside a union when emulating MSVC 1900+ even if `fms-extensions` is enabled.
   Starting with VS2015, MSVC 1900, this Microsoft extension is no longer allowed and always results in an error.
   Clang now follows the MSVC behavior in this scenario.

clang/include/clang/Driver/Options.td

@@ -8531,7 +8531,7 @@ def _SLASH_execution_charset : CLCompileJoined<"execution-charset:">,
   HelpText<"Set runtime encoding, supports only UTF-8">,
   Alias<fexec_charset_EQ>;
 def _SLASH_std : CLCompileJoined<"std:">,
-  HelpText<"Set language version (c++14,c++17,c++20,c++latest,c11,c17)">;
+  HelpText<"Set language version (c++14,c++17,c++20,c++23preview,c++latest,c11,c17)">;
 def _SLASH_U : CLJoinedOrSeparate<"U">, HelpText<"Undefine macro">,
   MetaVarName<"<macro>">, Alias<U>;
 def _SLASH_validate_charset : CLFlag<"validate-charset">,

clang/lib/Basic/Targets/OSTargets.cpp

@@ -214,9 +214,11 @@ static void addVisualCDefines(const LangOptions &Opts, MacroBuilder &Builder) {
       Builder.defineMacro("_HAS_CHAR16_T_LANGUAGE_SUPPORT", Twine(1));
 
     if (Opts.isCompatibleWithMSVC(LangOptions::MSVC2015)) {
-      if (Opts.CPlusPlus23)
+      if (Opts.CPlusPlus26)
         // TODO update to the proper value.
-        Builder.defineMacro("_MSVC_LANG", "202004L");
+        Builder.defineMacro("_MSVC_LANG", "202400L");
+      else if (Opts.CPlusPlus23)
+        Builder.defineMacro("_MSVC_LANG", "202302L");
       else if (Opts.CPlusPlus20)
         Builder.defineMacro("_MSVC_LANG", "202002L");
       else if (Opts.CPlusPlus17)

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7225,6 +7225,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                              .Case("c++17", "-std=c++17")
                              .Case("c++20", "-std=c++20")
                              // TODO add c++23 and c++26 when MSVC supports it.
+                             .Case("c++23preview", "-std=c++23")
                              .Case("c++latest", "-std=c++26")
                              .Default("");
       if (LanguageStandard.empty())

clang/test/Driver/cl-options.c

@@ -605,6 +605,9 @@
 // RUN: %clang_cl -fmsc-version=1900 -TP -std:c++20 -### -- %s 2>&1 | FileCheck -check-prefix=STDCXX20 %s
 // STDCXX20: -std=c++20
 
+// RUN: %clang_cl -fmsc-version=1900 -TP -std:c++23preview -### -- %s 2>&1 | FileCheck -check-prefix=STDCXX23PREVIEW %s
+// STDCXX23PREVIEW: -std=c++23
+
 // RUN: %clang_cl -fmsc-version=1900 -TP -std:c++latest -### -- %s 2>&1 | FileCheck -check-prefix=STDCXXLATEST %s
 // STDCXXLATEST: -std=c++26
 

clang/test/Preprocessor/predefined-win-macros.c

@@ -56,7 +56,12 @@
 // RUN: %clang_cc1 %s -x c++ -E -dM -triple i686-pc-win32 -fms-extensions -fms-compatibility \
 // RUN:     -fms-compatibility-version=19.00 -std=c++23 -o - | FileCheck -match-full-lines %s --check-prefix=CHECK-MS-CPP2B
 // CHECK-MS-CPP2B: #define _MSC_VER 1900
-// CHECK-MS-CPP2B: #define _MSVC_LANG 202004L
+// CHECK-MS-CPP2B: #define _MSVC_LANG 202302L
+
+// RUN: %clang_cc1 %s -x c++ -E -dM -triple i686-pc-win32 -fms-extensions -fms-compatibility \
+// RUN:     -fms-compatibility-version=19.00 -std=c++26 -o - | FileCheck -match-full-lines %s --check-prefix=CHECK-MS-CPP2C
+// CHECK-MS-CPP2C: #define _MSC_VER 1900
+// CHECK-MS-CPP2C: #define _MSVC_LANG 202400L
 
 // RUN: %clang_cc1 -triple i386-windows %s -E -dM -o - \
 // RUN:   | FileCheck -match-full-lines %s --check-prefix=CHECK-X86-WIN