refine noexcept specifiers on constructors

[lightmare]

Refs #61

A couple additions to detail::value_traits:

bool is_direct = direct_index != invalid_value;
bool is_valid = index != invalid_value;
typedef value_type = remove_reference<T>; // do it here so there's less clutter on use
typedef target_type = tuple_element<index, tuple<Types...>>

Because of the target_type, instantiation of value_traits<T, U> fails if !is_convertible<T, U>. I use this for enabling conversion constructor overloads.

I had to comment two tests because of that, I don't know how to test for "substitution failure must be an error". I tried a few things, but always ended up with invalid use of incomplete type in std::tuple_element<0ul, std::tuple<>>.

[lightmare]

Hm, so gcc 4.7 doesn't have ./variant.hpp:831:25: error: ‘is_nothrow_destructible’ is not a member of ‘std’. That can be wrapped in #ifdef.

But MSVC has more trouble. Question is: is it the compiler's failure to understand SFINAE, or am I using it wrong? Here's the line where Traits substitution fails (on purpose) if T is not convertible to any stored type: https://github.com/mapbox/variant/pull/62/files#diff-dab208458c807c54b530c13b19c80187R589 . Does anyone know whether deferring the failure until Traits::target_type is needed in noexcept would make MSVC happy?

[artemp]

Hm, so gcc 4.7 doesn't have ./variant.hpp:831:25: error: ‘is_nothrow_destructible’ is not a member of ‘std’. That can be wrapped in #ifdef.

std::is_nothrow_destructible is part of C++11 and this only means gcc 4.7 doesn't confirm to the standard

But MSVC has more trouble.

Which version are you using ? It seems to work on AppVeyor
/cc @lightmare @joto

[artemp]

Overall looks like move in the right direction thanks again @lightmare.

I'm inclined to merge this PR as it's definitely an improvement from just having noexcept :)

[lightmare]

Which version are you using ? It seems to work on AppVeyor

I'm not using MSVC. The original commit failed on AppVeyor, I fixed that in cfa1f95

Ah I missed it, great !