[PATCH] [libcxx] Fix copy/move issues caused by __tuple_leaf's converting constructor.

[Eric Fiselier]

Hi mclow.lists,

This patch contains two fixes issues arising around __tuple_leafs converting constructor.

1. The first problem is that __tuple_leaf<T> uses is_constructible<T, U> to enable/disable it's converting constructor `__tuple_leaf(U &&)`. Evaluating is_constructible under certain conditions can cause a hard compile error. Currently is_constructible<T, __tuple_leaf> is evaluated whenever __tuple_leaf is copied or moved. This patch prevents that from happening by only evaluating is_constructible when U != __tuple_leaf.

2. The second problem is that __tuple_leaf's specialization for empty types does have an implicitly generated move constructor because it declares a deleted assignment operator.
This causes the converting to be constructor during moves. The problem is solved by disabling the constructor when U == __tuple_leaf and giving __tuple_leaf a defaulted move constructor (so the defaulted copy constructor is not called). 

There is also a small fix to two tests that needed extra brackets around an assert macro.

http://reviews.llvm.org/D4451

Files:
  include/__tuple
  include/tuple
  test/utilities/tuple/tuple.tuple/tuple.cnstr/const_Types.pass.cpp
  test/utilities/tuple/tuple.tuple/tuple.cnstr/move.pass.cpp
  test/utilities/tuple/tuple.tuple/tuple.elem/tuple.by.type1.fail.cpp
  test/utilities/utility/pairs/pair.astuple/pairs.by.type1.fail.cpp
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