[Bug ipa/61659] [4.9/4.10 Regression] Extra undefined symbol because of devirtualization

["rafael.espindola at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61659

--- Comment #15 from Rafael Avila de Espindola <rafael.espindola at gmail dot com> ---
(In reply to Jason Merrill from comment #14)
> Right.  My patch causes us to synthesize ~I so that it's available for
> devirtualization, which we weren't doing before.  This is allowed by the C++
> standard: 3.2p3 says "A virtual member function is odr-used if it is not
> pure." so the compiler is free to instantiate/synthesize any virtual
> function.
> 
> Avoiding this while still getting the devirtualization benefit would require
> some mechanism for devirtualization to call back into the front end to
> trigger the instantiation/synthesis directly.  This might be workable for an
> individual translation unit, but not for LTO.

And make which c++ errors user see dependent on which optimizations they run,
which is not very friendly. I agree your approach is better.

Markus Trippelsdorf:

The include should go where it is used, so in GCMetadata.h.