Sema: Disallow derived classes with virtual bases from having flexible array members

Flexible array members only work out if they are the last field of a
record, however virtual bases would give us many situations where the
flexible array member would overlap with the virtual base fields.

It is unlikely in the extreme that this behavior was intended by the
user so raise a diagnostic instead of accepting.  This is will not
reject conforming code because flexible array members are an extension
in C++ mode.

llvm-svn: 193920

3 files changed, 16 insertions, 0 deletions

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index ae2d9c9e779..86108be0b93 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3937,6 +3937,9 @@ def ext_variable_sized_type_in_struct : ExtWarn<
 
 def ext_c99_flexible_array_member : Extension<
   "flexible array members are a C99 feature">, InGroup<C99>;
+def err_flexible_array_virtual_base : Error<
+  "flexible array member %0 not allowed in "
+  "%select{struct|interface|union|class|enum}1 which has a virtual base class">;
 def err_flexible_array_empty_aggregate : Error<
   "flexible array member %0 not allowed in otherwise empty "
   "%select{struct|interface|union|class|enum}1 is not allowed">;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 101f4ce8988..d9dd4507de6 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -11841,6 +11841,15 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
       if (DiagID)
         Diag(FD->getLocation(), DiagID) << FD->getDeclName()
                                         << Record->getTagKind();
+      // While the layout of types that contain virtual bases is not specified
+      // by the C++ standard, both the Itanium and Microsoft C++ ABIs place
+      // virtual bases after the derived members.  This would make a flexible
+      // array member declared at the end of an object not adjacent to the end
+      // of the type.
+      if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Record))
+        if (RD->getNumVBases() != 0)
+          Diag(FD->getLocation(), diag::err_flexible_array_virtual_base)
+            << FD->getDeclName() << Record->getTagKind();
       if (!getLangOpts().C99)
         Diag(FD->getLocation(), diag::ext_c99_flexible_array_member)
           << FD->getDeclName() << Record->getTagKind();
diff --git a/clang/test/SemaCXX/flexible-array-test.cpp b/clang/test/SemaCXX/flexible-array-test.cpp
index e6c3132801f..f287711eeb6 100644
--- a/clang/test/SemaCXX/flexible-array-test.cpp
+++ b/clang/test/SemaCXX/flexible-array-test.cpp
@@ -66,4 +66,8 @@ struct Storage : StorageBase {
   int data[];
 };
 
+struct VirtStorage : virtual StorageBase {
+  int data[]; // expected-error {{flexible array member 'data' not allowed in struct which has a virtual base class}}
+};
+
 }