[Sema] Union(s) cannot have virtual functions

[davide]

Summary

This fixes PR23931 .
I'm not entirely sure about the words of the error message.

[davide]

Example after patching:

$ clang++ union.cpp -o union
union.cpp:2:3: error: function 'f' declared 'virtual' within a union

virtual void f(...);
^

1 error generated.

g++ output, for reference

$ g++5 union.cpp -o union
union.cpp:2:21: error: function 'f' declared virtual inside a union

virtual void f(...);
                  ^

[rsmith]

LGTM with minor tweaks.

include/clang/Basic/DiagnosticSemaKinds.td

1266

This diagnostic is true, but doesn't actually explain what the problem is to someone unaware of the language rule. Also, listing the function name doesn't seem useful as the diagnostic is pointing at the function. Would just "unions cannot have virtual functions" work for you?

lib/Sema/SemaDecl.cpp

7211

Do we need to mark the function as invalid? It seems like we should be able to recover from this case fine, without any follow-on problems. (In fact, it's a little disturbing how /well/ we support unions with virtual functions today...)

[davide]

Tweaked after your suggestions and committed as r240889.

include/clang/Basic/DiagnosticSemaKinds.td

 def err_mutable_function : Error<"'mutable' cannot be applied to functions">;
 def err_mutable_reference : Error<"'mutable' cannot be applied to references">;
 def ext_mutable_reference : ExtWarn<
   "'mutable' on a reference type is a Microsoft extension">,
   InGroup<Microsoft>;
 def err_mutable_const : Error<"'mutable' and 'const' cannot be mixed">;
 def err_mutable_nonmember : Error<
   "'mutable' can only be applied to member variables">;
+def err_virtual_in_union : Error<
+  "function %0 declared 'virtual' within a union">;
 def err_virtual_non_function : Error<
   "'virtual' can only appear on non-static member functions">;
 def err_virtual_out_of_class : Error<
   "'virtual' can only be specified inside the class definition">;
 def err_virtual_member_function_template : Error<
   "'virtual' cannot be specified on member function templates">;
 def err_static_overrides_virtual : Error<
   "'static' member function %0 overrides a virtual function in a base class">;

lib/Sema/SemaDecl.cpp

 
     // If this is a method defined in an __interface, and is not a constructor
     // or an overloaded operator, then set the pure flag (isVirtual will already
     // return true).
     if (const CXXRecordDecl *Parent =
           dyn_cast<CXXRecordDecl>(NewFD->getDeclContext())) {
       if (Parent->isInterface() && cast<CXXMethodDecl>(NewFD)->isUserProvided())
         NewFD->setPure(true);
+
+      // C++ [class.union]p2
+      //   A union can have member functions, but not virtual functions.
+      if (isVirtual && Parent->isUnion()) {
+        Diag(D.getDeclSpec().getVirtualSpecLoc(),
+            diag::err_virtual_in_union) << NewFD->getDeclName();
+        NewFD->setInvalidDecl();
+      }
     }
 
     SetNestedNameSpecifier(NewFD, D);
     isExplicitSpecialization = false;
     isFunctionTemplateSpecialization = false;
     if (D.isInvalidType())
       NewFD->setInvalidDecl();
 

test/SemaCXX/virtual-function-in-union.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+union x {
+  virtual void f(); // expected-error {{function 'f' declared 'virtual' within a union}}
+};