[libc++] Try and prevent evaluation of `is_default_constructible` on tuples default constructor if it is not needed.
Summary:
Currently parts of the SFINAE on tuples default constructor always gets evaluated even when the default constructor is never called or instantiated. This can cause a hard compile error when a tuple is created with types that do not have a default constructor. Below is a self contained example using a pair like class. This code will not compile but probably should.
```
#include <type_traits>
template <class T>
struct IllFormedDefaultImp {
IllFormedDefaultImp(T x) : value(x) {}
constexpr IllFormedDefaultImp() {}
T value;
};
typedef IllFormedDefaultImp<int &> IllFormedDefault;
template <class T, class U>
struct pair
{
template <bool Dummy = true,
class = typename std::enable_if<
std::is_default_constructible<T>::value
&& std::is_default_constructible<U>::value
&& Dummy>::type
>
constexpr pair() : first(), second() {}
pair(T const & t, U const & u) : first(t), second(u) {}
T first;
U second;
};
int main()
{
int x = 1;
IllFormedDefault v(x);
pair<IllFormedDefault, IllFormedDefault> p(v, v);
}
```
One way to fix this is to use `Dummy` in a more involved way in the constructor SFINAE. The following patch fixes these sorts of hard compile errors for tuple.
Reviewers: mclow.lists, rsmith, K-ballo, EricWF
Reviewed By: EricWF
Subscribers: ldionne, cfe-commits
Differential Revision: http://reviews.llvm.org/D7569
llvm-svn: 230120
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 65500d4b29bdd03f6e3d14bd09550465bd9e16ed
diff --git a/include/tuple b/include/tuple
index 93518d8..21fa900 100644
--- a/include/tuple
+++ b/include/tuple
@@ -511,8 +511,8 @@
typename tuple_element<_Jp, tuple<_Up...> >::type&& get(tuple<_Up...>&&) _NOEXCEPT;
public:
- template <bool _Dummy = true, class _Up = typename enable_if<
- __all<(_Dummy && is_default_constructible<_Tp>::value)...>::value
+ template <bool _Dummy = true, class = typename enable_if<
+ __all<__dependent_type<is_default_constructible<_Tp>, _Dummy>::value...>::value
>::type>
_LIBCPP_INLINE_VISIBILITY
_LIBCPP_CONSTEXPR tuple()