[c++2a] Add option -std=c++2a to enable support for potential/transitional C++2a features

[faisalv]

Summary

At the optimistic least, I'm hoping to get around to adding support for familiar template syntax to lambdas and vaopt to the preprocessor, under it.

[faisalv]

Remove some erroneously included fragments (patches to Sema) of a future potential patch.

[aaron.ballman]

Has GCC picked the same name for their language standard? I want to make sure we're consistent there (and think this is the correct name).

[faisalv]

Hmm - good point - not sure about the best way to confirm that...

I'm fairly sure Jason said something during a discussion with me (about the additional template syntax to lambdas) in which he referred to gcc as already having a "C++2a feature" - and so i just inferred that that's what they were calling it ...

[aaron.ballman]

LGTM with a small commenting nit.

lib/Frontend/InitPreprocessor.cpp

377

Missing full stop at the end of the comment.

[faisalv]

Committed here https://reviews.llvm.org/rL308118

Thanks!

include/clang/Basic/LangOptions.def

 LANGOPT(CPlusPlus11       , 1, 0, "C++11")
 LANGOPT(CPlusPlus14       , 1, 0, "C++14")
 LANGOPT(CPlusPlus1z       , 1, 0, "C++1z")
+LANGOPT(CPlusPlus2a       , 1, 0, "C++2a")
 LANGOPT(ObjC1             , 1, 0, "Objective-C 1")
 LANGOPT(ObjC2             , 1, 0, "Objective-C 2")
 BENIGN_LANGOPT(ObjCDefaultSynthProperties , 1, 0,

include/clang/Frontend/LangStandard.h

   CPlusPlus11 = (1 << 4),
   CPlusPlus14 = (1 << 5),
   CPlusPlus1z = (1 << 6),
-  Digraphs = (1 << 7),
-  GNUMode = (1 << 8),
-  HexFloat = (1 << 9),
-  ImplicitInt = (1 << 10),
-  OpenCL = (1 << 11)
+  CPlusPlus2a = (1 << 7),
+  Digraphs = (1 << 8),
+  GNUMode = (1 << 9),
+  HexFloat = (1 << 10),
+  ImplicitInt = (1 << 11),
+  OpenCL = (1 << 12)
 };
 
 }

   /// isCPlusPlus1z - Language is a C++17 variant (or later).
   bool isCPlusPlus1z() const { return Flags & frontend::CPlusPlus1z; }
 
+  /// isCPlusPlus2a - Language is a post-C++17 variant (or later).
+  bool isCPlusPlus2a() const { return Flags & frontend::CPlusPlus2a; }
+
+
   /// hasDigraphs - Language supports digraphs.
   bool hasDigraphs() const { return Flags & frontend::Digraphs; }
 

include/clang/Frontend/LangStandards.def

              LineComment | CPlusPlus | CPlusPlus11 | CPlusPlus14 | CPlusPlus1z |
              Digraphs | HexFloat | GNUMode)
 
+LANGSTANDARD(cxx2a, "c++2a",
+             CXX, "Working draft for ISO C++ 2020",
+             LineComment | CPlusPlus | CPlusPlus11 | CPlusPlus14 | CPlusPlus1z |
+             CPlusPlus2a | Digraphs | HexFloat)
+
+LANGSTANDARD(gnucxx2a, "gnu++2a",
+             CXX, "Working draft for ISO C++ 2020 with GNU extensions",
+             LineComment | CPlusPlus | CPlusPlus11 | CPlusPlus14 | CPlusPlus1z |
+             CPlusPlus2a | Digraphs | HexFloat | GNUMode)
+
 // OpenCL
 LANGSTANDARD(opencl10, "cl1.0",
              OpenCL, "OpenCL 1.0",

lib/Frontend/CompilerInvocation.cpp

   Opts.CPlusPlus11 = Std.isCPlusPlus11();
   Opts.CPlusPlus14 = Std.isCPlusPlus14();
   Opts.CPlusPlus1z = Std.isCPlusPlus1z();
+  Opts.CPlusPlus2a = Std.isCPlusPlus2a();
   Opts.Digraphs = Std.hasDigraphs();
   Opts.GNUMode = Std.isGNUMode();
   Opts.GNUInline = !Opts.C99 && !Opts.CPlusPlus;

lib/Frontend/InitPreprocessor.cpp

     else if (!LangOpts.GNUMode && LangOpts.Digraphs)
       Builder.defineMacro("__STDC_VERSION__", "199409L");
   } else {
+    // FIXME: Use correct value for C++20
+    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.
-    if (LangOpts.CPlusPlus1z)
+    else if (LangOpts.CPlusPlus1z)
       Builder.defineMacro("__cplusplus", "201703L");
     // C++1y [cpp.predefined]p1:
     //   The name __cplusplus is defined to the value 201402L when compiling a

lib/Sema/SemaDeclCXX.cpp

       //   Local variables shall not be used in default argument
       //   expressions.
       if (VDecl->isLocalVarDecl())
-        return S->Diag(DRE->getLocStart(),
-                       diag::err_param_default_argument_references_local)
-          << VDecl->getDeclName() << DefaultArg->getSourceRange();
+        if (!VDecl->isStaticLocal() &&
+            !(VDecl->isUsableInConstantExpressions(VDecl->getASTContext()) &&
+              VDecl->checkInitIsICE()))
+          return S->Diag(DRE->getLocStart(),
+                         diag::err_param_default_argument_references_local)
+                 << VDecl->getDeclName() << DefaultArg->getSourceRange();
     }
 
     return false;

lib/Sema/SemaExpr.cpp

   // We already type-checked the argument, so we know it works. 
   // Just mark all of the declarations in this potentially-evaluated expression
   // as being "referenced".
-  MarkDeclarationsReferencedInExpr(Param->getDefaultArg(),
-                                   /*SkipLocalVariables=*/true);
+  MarkDeclarationsReferencedInExpr(Param->getDefaultArg());
   return false;
 }
 

test/Driver/std.cpp

 // 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
 
+
 void f(int n) {
   typeof(n)();
   decltype(n)();

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

test/Driver/unknown-std.cpp

 // CHECK-NEXT: note: use 'gnu++14' for 'ISO C++ 2014 with amendments and GNU extensions' standard
 // CHECK-NEXT: note: use 'c++1z' for 'Working draft for ISO C++ 2017' standard
 // CHECK-NEXT: note: use 'gnu++1z' for 'Working draft for ISO C++ 2017 with GNU extensions' standard
+// CHECK-NEXT: note: use 'c++2a' for 'Working draft for ISO C++ 2020' standard
+// CHECK-NEXT: note: use 'gnu++2a' for 'Working draft for ISO C++ 2020 with GNU extensions' standard
 // CUDA-NEXT: note: use 'cuda' for 'NVIDIA CUDA(tm)' standard
 
 // Make sure that no other output is present.

test/Preprocessor/init.c

 // BLOCKS:#define __block __attribute__((__blocks__(byref)))
 //
 //
+// RUN: %clang_cc1 -x c++ -std=c++2a -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix CXX2A %s
+//
+// CXX2A:#define __GNUG__ {{.*}}
+// CXX2A:#define __GXX_EXPERIMENTAL_CXX0X__ 1
+// CXX2A:#define __GXX_RTTI 1
+// CXX2A:#define __GXX_WEAK__ 1
+// CXX2A:#define __cplusplus 201707L
+// CXX2A:#define __private_extern__ extern
+//
 // RUN: %clang_cc1 -x c++ -std=c++1z -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix CXX1Z %s
 //
 // CXX1Z:#define __GNUG__ {{.*}}

 // 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++ -std=gnu++2a -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX2A %s
 //
+// GXX2A:#define __GNUG__ {{.*}}
+// GXX2A:#define __GXX_WEAK__ 1
+// GXX2A:#define __cplusplus 201707L
+// GXX2A:#define __private_extern__ extern
+//
+//
 // RUN: %clang_cc1 -x c++ -std=gnu++1z -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GXX1Z %s
 //
 // GXX1Z:#define __GNUG__ {{.*}}