[v3] "SFINAE-friendly" std::common_type and more

[Daniel Krügler]

2012/10/10 Paolo Carlini <paolo.carlini@oracle.com>:
> ... tested x86_64-linux. Committed to mainline.

I'd like to mention that this patch does not reflect what the core
language says, e.g.

static_assert(is_type<std::common_type<int&&, int&&>, int>(), "");

Should assert, the correct result being int&&. The test currently
holds because of compiler bug 53000.

- Daniel