GCC Compatibility: Support for __final specifier

[erichkeane]

Summary

As reported in bug 28473, GCC supports ‘final’ functionality in pre-C++11 code using the __final keyword. Clang currently supports the ‘final’ keyword in accordance with the C++11 specification, however it ALSO supports it in pre-C++11 mode, with a warning.

This patch adds the ‘__final’ keyword for compatibility with GCC in GCC Keywords mode (so it is enabled with existing flags), and issues a warning on its usage (suggesting switching to the C++11 keyword). This patch also adds a regression test for the functionality described. I believe this patch has minimal impact, as it simply adds a new keyword for existing behavior.

This has been validated with check-clang to avoid regressions. Patch is created in reference to revisions 276665

[majnemer]

LGTM as-is, clang's behavior would be congruent with GCC's.

lib/Parse/ParseDeclCXX.cpp

3024–3026

Please remove these braces.

[erichkeane]

Fixed the braces mentioned.

include/clang/Basic/DiagnosticSemaKinds.td

   "'%0' qualifier on omitted return type %1 has no effect">,
   InGroup<IgnoredQualifiers>;
 
+def ext_warn_gnu_final : ExtWarn<
+  "__final is a GNU extension, consider using C++11 final">,
+  InGroup<GccCompat>;
+
 } // end of sema component.

include/clang/Parse/Parser.h

 
   /// C++0x contextual keywords.
   mutable IdentifierInfo *Ident_final;
+  mutable IdentifierInfo *Ident_GNU_final;
   mutable IdentifierInfo *Ident_override;
 
   // C++ type trait keywords that can be reverted to identifiers and still be

include/clang/Sema/DeclSpec.h

     VS_None = 0,
     VS_Override = 1,
     VS_Final = 2,
-    VS_Sealed = 4
+    VS_Sealed = 4,
+    // Represents the __final keyword, which is legal for gcc in pre-C++11 mode.
+    VS_GNU_Final = 8
   };
 
   VirtSpecifiers() : Specifiers(0), LastSpecifier(VS_None) { }

   bool isOverrideSpecified() const { return Specifiers & VS_Override; }
   SourceLocation getOverrideLoc() const { return VS_overrideLoc; }
 
-  bool isFinalSpecified() const { return Specifiers & (VS_Final | VS_Sealed); }
+  bool isFinalSpecified() const { return Specifiers & (VS_Final | VS_Sealed | VS_GNU_Final); }
   bool isFinalSpelledSealed() const { return Specifiers & VS_Sealed; }
   SourceLocation getFinalLoc() const { return VS_finalLoc; }
 

lib/Parse/ParseDeclCXX.cpp

 ///       virt-specifier:
 ///         override
 ///         final
+///         __final
 VirtSpecifiers::Specifier Parser::isCXX11VirtSpecifier(const Token &Tok) const {
   if (!getLangOpts().CPlusPlus || Tok.isNot(tok::identifier))
     return VirtSpecifiers::VS_None;

   // Initialize the contextual keywords.
   if (!Ident_final) {
     Ident_final = &PP.getIdentifierTable().get("final");
+    if (getLangOpts().GNUKeywords)
+      Ident_GNU_final = &PP.getIdentifierTable().get("__final");
     if (getLangOpts().MicrosoftExt)
       Ident_sealed = &PP.getIdentifierTable().get("sealed");
     Ident_override = &PP.getIdentifierTable().get("override");

   if (II == Ident_final)
     return VirtSpecifiers::VS_Final;
 
+  if (II == Ident_GNU_final)
+    return VirtSpecifiers::VS_GNU_Final;
+
   return VirtSpecifiers::VS_None;
 }
 

         << VirtSpecifiers::getSpecifierName(Specifier);
     } else if (Specifier == VirtSpecifiers::VS_Sealed) {
       Diag(Tok.getLocation(), diag::ext_ms_sealed_keyword);
+    } else if (Specifier == VirtSpecifiers::VS_GNU_Final) {
+      Diag(Tok.getLocation(), diag::ext_warn_gnu_final);
     } else {
       Diag(Tok.getLocation(),
            getLangOpts().CPlusPlus11

 bool Parser::isCXX11FinalKeyword() const {
   VirtSpecifiers::Specifier Specifier = isCXX11VirtSpecifier();
   return Specifier == VirtSpecifiers::VS_Final ||
+         Specifier == VirtSpecifiers::VS_GNU_Final || 
          Specifier == VirtSpecifiers::VS_Sealed;
 }
 

   if (getLangOpts().CPlusPlus && Tok.is(tok::identifier)) {
     VirtSpecifiers::Specifier Specifier = isCXX11VirtSpecifier(Tok);
     assert((Specifier == VirtSpecifiers::VS_Final ||
+            Specifier == VirtSpecifiers::VS_GNU_Final || 
             Specifier == VirtSpecifiers::VS_Sealed) &&
            "not a class definition");
     FinalLoc = ConsumeToken();

         << VirtSpecifiers::getSpecifierName(Specifier);
     else if (Specifier == VirtSpecifiers::VS_Sealed)
       Diag(FinalLoc, diag::ext_ms_sealed_keyword);
+    else if (Specifier == VirtSpecifiers::VS_GNU_Final) {
+      Diag(FinalLoc, diag::ext_warn_gnu_final);
+    }
 
     // Parse any C++11 attributes after 'final' keyword.
     // These attributes are not allowed to appear here,

lib/Sema/DeclSpec.cpp

   switch (VS) {
   default: llvm_unreachable("Unknown specifier!");
   case VS_Override: VS_overrideLoc = Loc; break;
+  case VS_GNU_Final:
   case VS_Sealed:
   case VS_Final:    VS_finalLoc = Loc; break;
   }

   default: llvm_unreachable("Unknown specifier");
   case VS_Override: return "override";
   case VS_Final: return "final";
+  case VS_GNU_Final: return "__final";
   case VS_Sealed: return "sealed";
   }
 }

test/Parser/gcc-__final-compatibility.cpp

+// RUN: %clang_cc1 -std=c++98 -fgnu-keywords -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++11 -fgnu-keywords -fsyntax-only -verify %s
+
+struct B {
+  virtual void g();
+};
+struct D __final : B { // expected-warning {{__final is a GNU extension, consider using C++11 final}}
+  virtual void g() __final; // expected-warning {{__final is a GNU extension, consider using C++11 final}}
+};