Add -std=c++20 flag, replace C++2a with C++20 throughout the Clang

user interface and documentation, and update __cplusplus for C++20.

WG21 considers the C++20 standard to be finished (even though it still
has some more steps to pass through in the ISO process).

The old flag names are accepted for compatibility, as usual, and we
still have lots of references to C++2a in comments and identifiers;
those can be cleaned up separately.

(cherry picked from commit 24ad121582454e625bdad125c90d9ac0dae948c8)

1 files changed, 27 insertions, 17 deletions

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 8a0ff55e44f..c273cb96d9b 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -344,13 +344,27 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
                                                const LangOptions &LangOpts,
                                                const FrontendOptions &FEOpts,
                                                MacroBuilder &Builder) {
+  // C++ [cpp.predefined]p1:
+  //   The following macro names shall be defined by the implementation:
+
+  //   -- __STDC__
+  //      [C++] Whether __STDC__ is predefined and if so, what its value is,
+  //      are implementation-defined.
+  // (Removed in C++20.)
   if (!LangOpts.MSVCCompat && !LangOpts.TraditionalCPP)
     Builder.defineMacro("__STDC__");
+  //   -- __STDC_HOSTED__
+  //      The integer literal 1 if the implementation is a hosted
+  //      implementation or the integer literal 0 if it is not.
   if (LangOpts.Freestanding)
     Builder.defineMacro("__STDC_HOSTED__", "0");
   else
     Builder.defineMacro("__STDC_HOSTED__");
 
+  //   -- __STDC_VERSION__
+  //      [C++] Whether __STDC_VERSION__ is predefined and if so, what its
+  //      value is, are implementation-defined.
+  // (Removed in C++20.)
   if (!LangOpts.CPlusPlus) {
     if (LangOpts.C17)
       Builder.defineMacro("__STDC_VERSION__", "201710L");
@@ -361,33 +375,29 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
     else if (!LangOpts.GNUMode && LangOpts.Digraphs)
       Builder.defineMacro("__STDC_VERSION__", "199409L");
   } else {
-    // FIXME: Use correct value for C++20.
+    //   -- __cplusplus
+    //      [C++20] The integer literal 202002L.
     if (LangOpts.CPlusPlus2a)
-      Builder.defineMacro("__cplusplus", "201707L");
-    // C++17 [cpp.predefined]p1:
-    //   The name __cplusplus is defined to the value 201703L when compiling a
-    //   C++ translation unit.
+      Builder.defineMacro("__cplusplus", "202002L");
+    //      [C++17] The integer literal 201703L.
     else if (LangOpts.CPlusPlus17)
       Builder.defineMacro("__cplusplus", "201703L");
-    // C++1y [cpp.predefined]p1:
-    //   The name __cplusplus is defined to the value 201402L when compiling a
-    //   C++ translation unit.
+    //      [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");
-    // C++11 [cpp.predefined]p1:
-    //   The name __cplusplus is defined to the value 201103L when compiling a
-    //   C++ translation unit.
+    //      [C++11] The name __cplusplus is defined to the value 201103L when
+    //      compiling a C++ translation unit.
     else if (LangOpts.CPlusPlus11)
       Builder.defineMacro("__cplusplus", "201103L");
-    // C++03 [cpp.predefined]p1:
-    //   The name __cplusplus is defined to the value 199711L when compiling a
-    //   C++ translation unit.
+    //      [C++03] The name __cplusplus is defined to the value 199711L when
+    //      compiling a C++ translation unit.
     else
       Builder.defineMacro("__cplusplus", "199711L");
 
-    // C++1z [cpp.predefined]p1:
-    //   An integer literal of type std::size_t whose value is the alignment
-    //   guaranteed by a call to operator new(std::size_t)
+    //   -- __STDCPP_DEFAULT_NEW_ALIGNMENT__
+    //      [C++17] An integer literal of type std::size_t whose value is the
+    //      alignment guaranteed by a call to operator new(std::size_t)
     //
     // We provide this in all language modes, since it seems generally useful.
     Builder.defineMacro("__STDCPP_DEFAULT_NEW_ALIGNMENT__",