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

1 files changed, 9 insertions, 0 deletions

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();