Prohibit enum <-> int mixup

[Jonathan Wakely]

On 31 January 2011 19:16, Ian Lance Taylor wrote:
>
> Using -std=c++0x will cause libstdc++ to build in C++0x mode, which last
> time I checked was not strictly ABI compatible with libstdc++ not built
> in C++0x mode.  So if you link with a dynamic libstdc++.so, you may get
> in trouble in some complex scenarios.

Do you remember the details?  I thought we'd done a good job of not
breaking the ABI, and have avoided making some changes required by
C+0x because they change the ABI.  I'd like to document the potential
problems if you can remember where the incompatibilities are.