Chris Jefferson found this bug to pair/tuple introduced by a recent change to the draft that I missed.

llvm-svn: 126743
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b5d166ad93c9faf13fe5dfa15e94e5fe4a7abf11
diff --git a/include/__tuple b/include/__tuple
index 39e484a..b9f92c5 100644
--- a/include/__tuple
+++ b/include/__tuple
@@ -214,7 +214,7 @@
 template <class _Tp0, class ..._Tp, class _Up0, class ..._Up>
 struct __tuple_convertible_imp<true, __tuple_types<_Tp0, _Tp...>, __tuple_types<_Up0, _Up...> >
     : public integral_constant<bool,
-                               is_convertible<_Tp0, _Up0>::value &&
+                               is_constructible<_Up0, _Tp0>::value &&
                                __tuple_convertible_imp<true, __tuple_types<_Tp...>, __tuple_types<_Up...> >::value> {};
 
 template <>