[PATCH] [libc++] Try and prevent evaluation of `is_default_constructible` on tuples default constructor if it is not needed.

[Eric Fiselier]

Address @K-ballo's comments.


http://reviews.llvm.org/D7569

Files:
  include/tuple
  include/type_traits
  test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp

Index: include/tuple
===================================================================
--- include/tuple
+++ 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()
Index: include/type_traits
===================================================================
--- include/type_traits
+++ include/type_traits
@@ -216,6 +216,9 @@
 struct __void_t { typedef void type; };
 #endif
 
+template <class _Tp, bool>
+struct _LIBCPP_TYPE_VIS_ONLY __dependent_type : public _Tp {};
+
 template <bool _Bp, class _If, class _Then>
     struct _LIBCPP_TYPE_VIS_ONLY conditional {typedef _If type;};
 template <class _If, class _Then>
Index: test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp
===================================================================
--- test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp
+++ test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp
@@ -35,6 +35,16 @@
     ThrowingDefault() { }
 };
 
+struct IllFormedDefault {
+    IllFormedDefault(int x) : value(x) {}
+    template <bool Pred = false>
+    constexpr IllFormedDefault() {
+        static_assert(Pred,
+            "The default constructor should not be instantiated");
+    }
+    int value;
+};
+
 int main()
 {
     {
@@ -89,5 +99,12 @@
         assert(std::get<0>(t) == 0);
         assert(std::get<1>(t) == nullptr);
     }
+    {
+    // Check that the SFINAE on the default constructor is not evaluted when
+    // it isn't needed. If the default constructor is evaluted then this test
+    // should fail to compile.
+        IllFormedDefault v(0);
+        std::tuple<IllFormedDefault> t(v);
+    }
 #endif
 }

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