[cfe-dev] Fwd: anchoring explicit template instantiations

[David Blaikie]

[Forwarding to cfe-dev for the purpose of discussing what to do with
weak-vtables in explicit template instantiations - when/how should the
warning be emitted in this case, etc, details below]

For a bit of an experiment I've been trying to compile LLVM & Clang
with -Weverything (disabling any errors that seem like more noise/less
interesting). One warning I've recently hit a few instances of is
-Wweak-vtable which is, in fact, an explicitly documented LLVM coding
standard ( http://llvm.org/docs/CodingStandards.html#ll_virtual_anch
). Some instances of this have been easy to fix but a particular set of
them have been a little more problematic.

If you take a look at CommandLine.h/CommandLine.cpp you'll see some
code that basically amounts to this:

header:

   template<class DataType>
   class basic_parser {
     virtual ~basic_parser() {}
   };

   __extension__ extern template class basic_parser<bool>;

implementation:

   template class basic_parser<bool>;

(both lines are wrapped in a macro (Compiler.h:77-88) & are no-ops in
non-GNUC compilers (where the __extension__ extern is not available))

Adding in a virtual anchor function with an out-of-line (but still
template) definition (either in the header or the cpp file) does not
remove the warning.

So the question is - is there any way to anchor these explicit
instantiations? Should the warning (& possibly even the underlying
implementation/codegen) be fixed to not flag this particular case of
the GNUC extension - since these vtables should be able to be anchored
(with the addition of such an out of line definition - either in the
header or cpp file (though in this case I don't think it should be
necessary in the header - since only these explicit instantiations of
basic_parser are used))? Is there a portable way to address the
warning? If not, should the warning just be silent, or have a separate
group/warning for this case so the actionable warning can remain while
this one can be disabled?

Thanks,
- David