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

[Eric Fiselier]

Address Richard's comments. Create a generic `__dependent_type` template instead of using `__dependent_is_default_constructible`.


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
@@ -512,7 +512,7 @@
 public:
 
     template <bool _Dummy = true, class _Up = typename enable_if<
-        __all<(_Dummy && is_default_constructible<_Tp>::value)...>::value
+        __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
@@ -33,6 +33,15 @@
     ThrowingDefault() { }
 };
 
+template <class T>
+struct IllFormedDefaultImp {
+    IllFormedDefaultImp(T x) : value(x) {}
+    constexpr IllFormedDefaultImp() {}
+    T value;
+};
+
+typedef IllFormedDefaultImp<int &> IllFormedDefault;
+
 int main()
 {
     {
@@ -87,5 +96,13 @@
         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.
+        int x = 0;
+        IllFormedDefault v(x);
+        std::tuple<IllFormedDefault> t(v);
+    }
 #endif
 }

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