[c++2b] Add option -std=c++2b to enable support for potential C++2b features.

[rsmith]

There are some mechanical changes we should make to our documentation too, particularly www/cxx_status.html.

[curdeius]

In D92547#2430428, @rsmith wrote:

There are some mechanical changes we should make to our documentation too, particularly www/cxx_status.html.

Ok, I will soon send a patch to update cxx_status adding C++2b.

clang/include/clang/Basic/LangOptions.def

 LANGOPT(MicrosoftExt      , 1, 0, "Microsoft C++ extensions")
 LANGOPT(AsmBlocks         , 1, 0, "Microsoft inline asm blocks")
 LANGOPT(Borland           , 1, 0, "Borland extensions")
 LANGOPT(CPlusPlus         , 1, 0, "C++")
 LANGOPT(CPlusPlus11       , 1, 0, "C++11")
 LANGOPT(CPlusPlus14       , 1, 0, "C++14")
 LANGOPT(CPlusPlus17       , 1, 0, "C++17")
 LANGOPT(CPlusPlus20       , 1, 0, "C++20")
+LANGOPT(CPlusPlus2b       , 1, 0, "C++2b")
 LANGOPT(ObjC              , 1, 0, "Objective-C")
 BENIGN_LANGOPT(ObjCDefaultSynthProperties , 1, 0,
                "Objective-C auto-synthesized properties")
 BENIGN_LANGOPT(EncodeExtendedBlockSig , 1, 0,
                "Encoding extended block type signature")
 BENIGN_LANGOPT(ObjCInferRelatedResultType , 1, 1,
                "Objective-C related result type inference")
 LANGOPT(AppExt , 1, 0, "Objective-C App Extension")

clang/include/clang/Basic/LangStandard.h

enum LangFeatures {
   C11 = (1 << 2),
   C17 = (1 << 3),
   C2x = (1 << 4),
   CPlusPlus = (1 << 5),
   CPlusPlus11 = (1 << 6),
   CPlusPlus14 = (1 << 7),
   CPlusPlus17 = (1 << 8),
   CPlusPlus20 = (1 << 9),
-  Digraphs = (1 << 10),
-  GNUMode = (1 << 11),
-  HexFloat = (1 << 12),
-  ImplicitInt = (1 << 13),
-  OpenCL = (1 << 14)
+  CPlusPlus2b = (1 << 10),
+  Digraphs = (1 << 11),
+  GNUMode = (1 << 12),
+  HexFloat = (1 << 13),
+  ImplicitInt = (1 << 14),
+  OpenCL = (1 << 15)
 };
 
 /// LangStandard - Information about the properties of a particular language
 /// standard.
 struct LangStandard {
   enum Kind {
 #define LANGSTANDARD(id, name, lang, desc, features) \
     lang_##id,
public:
   bool isCPlusPlus14() const { return Flags & CPlusPlus14; }
 
   /// isCPlusPlus17 - Language is a C++17 variant (or later).
   bool isCPlusPlus17() const { return Flags & CPlusPlus17; }
 
   /// isCPlusPlus20 - Language is a C++20 variant (or later).
   bool isCPlusPlus20() const { return Flags & CPlusPlus20; }
 
+  /// isCPlusPlus2b - Language is a post-C++20 variant (or later).
+  bool isCPlusPlus2b() const { return Flags & CPlusPlus2b; }
+
   /// hasDigraphs - Language supports digraphs.
   bool hasDigraphs() const { return Flags & Digraphs; }
 
   /// isGNUMode - Language includes GNU extensions.
   bool isGNUMode() const { return Flags & GNUMode; }
 
   /// hasHexFloats - Language supports hexadecimal float constants.
   bool hasHexFloats() const { return Flags & HexFloat; }

clang/include/clang/Basic/LangStandards.def

 LANGSTANDARD_ALIAS_DEPR(cxx20, "c++2a")
 
 LANGSTANDARD(gnucxx20, "gnu++20",
              CXX, "ISO C++ 2020 DIS with GNU extensions",
              LineComment | CPlusPlus | CPlusPlus11 | CPlusPlus14 | CPlusPlus17 |
              CPlusPlus20 | Digraphs | HexFloat | GNUMode)
 LANGSTANDARD_ALIAS_DEPR(gnucxx20, "gnu++2a")
 
+LANGSTANDARD(cxx2b, "c++2b",
+             CXX, "Working draft for ISO C++ 2023 DIS",
+             LineComment | CPlusPlus | CPlusPlus11 | CPlusPlus14 | CPlusPlus17 |
+             CPlusPlus20 | CPlusPlus2b | Digraphs | HexFloat)
+
+LANGSTANDARD(gnucxx2b, "gnu++2b",
+             CXX, "Working draft for ISO C++ 2023 DIS with GNU extensions",
+             LineComment | CPlusPlus | CPlusPlus11 | CPlusPlus14 | CPlusPlus17 |
+             CPlusPlus20 | CPlusPlus2b | Digraphs | HexFloat | GNUMode)
+
 // OpenCL
 LANGSTANDARD(opencl10, "cl1.0",
              OpenCL, "OpenCL 1.0",
              LineComment | C99 | Digraphs | HexFloat | OpenCL)
 LANGSTANDARD_ALIAS_DEPR(opencl10, "cl")
 
 LANGSTANDARD(opencl11, "cl1.1",
              OpenCL, "OpenCL 1.1",

clang/lib/Frontend/CompilerInvocation.cpp

#endif
   Opts.C11 = Std.isC11();
   Opts.C17 = Std.isC17();
   Opts.C2x = Std.isC2x();
   Opts.CPlusPlus = Std.isCPlusPlus();
   Opts.CPlusPlus11 = Std.isCPlusPlus11();
   Opts.CPlusPlus14 = Std.isCPlusPlus14();
   Opts.CPlusPlus17 = Std.isCPlusPlus17();
   Opts.CPlusPlus20 = Std.isCPlusPlus20();
+  Opts.CPlusPlus2b = Std.isCPlusPlus2b();
   Opts.Digraphs = Std.hasDigraphs();
   Opts.GNUMode = Std.isGNUMode();
   Opts.GNUInline = !Opts.C99 && !Opts.CPlusPlus;
   Opts.GNUCVersion = 0;
   Opts.HexFloats = Std.hasHexFloats();
   Opts.ImplicitInt = Std.hasImplicitInt();
 
   // Set OpenCL Version.

clang/lib/Frontend/InitPreprocessor.cpp

if (!LangOpts.CPlusPlus) {
     else if (LangOpts.C11)
       Builder.defineMacro("__STDC_VERSION__", "201112L");
     else if (LangOpts.C99)
       Builder.defineMacro("__STDC_VERSION__", "199901L");
     else if (!LangOpts.GNUMode && LangOpts.Digraphs)
       Builder.defineMacro("__STDC_VERSION__", "199409L");
   } else {
     //   -- __cplusplus
+    // FIXME: Use correct value for C++23.
+    if (LangOpts.CPlusPlus2b)
+      Builder.defineMacro("__cplusplus", "202101L");
     //      [C++20] The integer literal 202002L.
-    if (LangOpts.CPlusPlus20)
+    else if (LangOpts.CPlusPlus20)
       Builder.defineMacro("__cplusplus", "202002L");
     //      [C++17] The integer literal 201703L.
     else if (LangOpts.CPlusPlus17)
       Builder.defineMacro("__cplusplus", "201703L");
     //      [C++14] The name __cplusplus is defined to the value 201402L when
     //      compiling a C++ translation unit.
     else if (LangOpts.CPlusPlus14)
       Builder.defineMacro("__cplusplus", "201402L");

clang/test/Driver/std.cpp

 // RUN: not %clang -std=c++98 %s -Wno-c++0x-compat -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX98 %s
 // RUN: not %clang -std=gnu++98 %s -Wno-c++0x-compat -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX98 %s
 // RUN: not %clang -std=c++03 %s -Wno-c++0x-compat -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX98 %s
 // RUN: not %clang -std=c++0x %s -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX11 %s
 // RUN: not %clang -std=gnu++0x %s -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX11 %s
 // RUN: not %clang -std=c++11 %s -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX11 %s
 // RUN: not %clang -std=gnu++11 %s -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX11 %s
 // RUN: not %clang -std=c++1y %s -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX1Y %s
 // RUN: not %clang -std=gnu++1y %s -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX1Y %s
 // RUN: not %clang -std=c++1z %s -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX1Z %s
 // RUN: not %clang -std=gnu++1z %s -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX1Z %s
 // RUN: not %clang -std=c++2a %s -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX2A %s
 // RUN: not %clang -std=gnu++2a %s -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX2A %s
-
+// RUN: not %clang -std=c++2b %s -fsyntax-only 2>&1 | FileCheck -check-prefix=CXX2B %s
+// RUN: not %clang -std=gnu++2b %s -fsyntax-only 2>&1 | FileCheck -check-prefix=GNUXX2B %s
 
 void f(int n) {
   typeof(n)();
   decltype(n)();
 }
 
 // CXX98: undeclared identifier 'typeof'
 // CXX98: undeclared identifier 'decltype'

 // GNUXX1Z-NOT: undeclared identifier 'decltype'
 
 // CXX2A: undeclared identifier 'typeof'
 // CXX2A-NOT: undeclared identifier 'decltype'
 
 // GNUXX2A-NOT: undeclared identifier 'typeof'
 // GNUXX2A-NOT: undeclared identifier 'decltype'
 
+// CXX2B: undeclared identifier 'typeof'
+// CXX2B-NOT: undeclared identifier 'decltype'
+
+// GNUXX2B-NOT: undeclared identifier 'typeof'
+// GNUXX2B-NOT: undeclared identifier 'decltype'

clang/test/Driver/unknown-std.cpp

 // CHECK-NEXT: note: use 'c++11' for 'ISO C++ 2011 with amendments' standard
 // CHECK-NEXT: note: use 'gnu++11' for 'ISO C++ 2011 with amendments and GNU extensions' standard
 // CHECK-NEXT: note: use 'c++14' for 'ISO C++ 2014 with amendments' standard
 // CHECK-NEXT: note: use 'gnu++14' for 'ISO C++ 2014 with amendments and GNU extensions' standard
 // CHECK-NEXT: note: use 'c++17' for 'ISO C++ 2017 with amendments' standard
 // CHECK-NEXT: note: use 'gnu++17' for 'ISO C++ 2017 with amendments and GNU extensions' standard
 // CHECK-NEXT: note: use 'c++20' for 'ISO C++ 2020 DIS' standard
 // CHECK-NEXT: note: use 'gnu++20' for 'ISO C++ 2020 DIS with GNU extensions' standard
+// CHECK-NEXT: note: use 'c++2b' for 'Working draft for ISO C++ 2023 DIS' standard
+// CHECK-NEXT: note: use 'gnu++2b' for 'Working draft for ISO C++ 2023 DIS with GNU extensions' standard
 // CUDA-NEXT: note: use 'cuda' for 'NVIDIA CUDA(tm)' standard
 
 // Make sure that no other output is present.
 // CHECK-NOT: {{^.+$}}
 

clang/test/Preprocessor/init.c

 // RUN: %clang_cc1 -E -dM -x assembler-with-cpp < /dev/null | FileCheck -match-full-lines -check-prefix ASM %s
 //
 // ASM:#define __ASSEMBLER__ 1
 //
 //
 // RUN: %clang_cc1 -fblocks -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix BLOCKS %s
 //
 // BLOCKS:#define __BLOCKS__ 1
 // BLOCKS:#define __block __attribute__((__blocks__(byref)))
 //
 //
+// RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=c++2b -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix CXX2B %s
+//
+// CXX2B:#define __GNUG__ 4
+// CXX2B:#define __GXX_EXPERIMENTAL_CXX0X__ 1
+// CXX2B:#define __GXX_RTTI 1
+// CXX2B:#define __GXX_WEAK__ 1
+// CXX2B:#define __cplusplus 202101L
+// CXX2B:#define __private_extern__ extern
+//
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=c++20 -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix CXX2A %s
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=c++2a -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix CXX2A %s
 //
 // CXX2A:#define __GNUG__ 4
 // CXX2A:#define __GXX_EXPERIMENTAL_CXX0X__ 1
 // CXX2A:#define __GXX_RTTI 1
 // CXX2A:#define __GXX_WEAK__ 1
 // CXX2A:#define __cplusplus 202002L

 // RUN: %clang_cc1 -E -dM -triple=x86_64-apple-darwin < /dev/null | FileCheck -match-full-lines -check-prefix C-DEFAULT %s
 // RUN: %clang_cc1 -E -dM -triple=armv7a-apple-darwin < /dev/null | FileCheck -match-full-lines -check-prefix C-DEFAULT %s
 //
 // C-DEFAULT:#define __STDC_VERSION__ 201710L
 //
 // RUN: %clang_cc1 -ffreestanding -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix FREESTANDING %s
 // FREESTANDING:#define __STDC_HOSTED__ 0
 //
+// RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=gnu++2b -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX2B %s
+//
+// GXX2B:#define __GNUG__ 4
+// GXX2B:#define __GXX_WEAK__ 1
+// GXX2B:#define __cplusplus 202101L
+// GXX2B:#define __private_extern__ extern
+//
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=gnu++20 -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX2A %s
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=gnu++2a -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX2A %s
 //
 // GXX2A:#define __GNUG__ 4
 // GXX2A:#define __GXX_WEAK__ 1
 // GXX2A:#define __cplusplus 202002L
 // GXX2A:#define __private_extern__ extern
 //