Fix tuple's conditionally explicit constructors for very weird user

types.

It seems some people like to write types that can explicitly convert
to anything, but cannot be used to explicitly construct anything.

This patch makes tuple tolerate such types, as is required
by the standard.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365074 91177308-0d34-0410-b5e6-96231b3b80d8

include/tuple

@@ -521,6 +521,13 @@ class _LIBCPP_TEMPLATE_VIS tuple
         template <class ..._Args>
         static constexpr bool __enable_implicit() {
             return
+               __tuple_constructible<
+                    tuple<_Args...>,
+                    typename __make_tuple_types<tuple,
+                             sizeof...(_Args) < sizeof...(_Tp) ?
+                                 sizeof...(_Args) :
+                                 sizeof...(_Tp)>::type
+                >::value &&
                 __tuple_convertible<
                     tuple<_Args...>,
                     typename __make_tuple_types<tuple,
@@ -547,7 +554,8 @@ class _LIBCPP_TEMPLATE_VIS tuple
     {
         template <class _Tuple>
         static constexpr bool __enable_implicit() {
-            return __tuple_convertible<_Tuple, tuple>::value;
+            return __tuple_constructible<_Tuple, tuple>::value
+                && __tuple_convertible<_Tuple, tuple>::value;
         }
 
         template <class _Tuple>
@@ -577,6 +585,7 @@ class _LIBCPP_TEMPLATE_VIS tuple
         template <class _Tuple>
         static constexpr bool __enable_implicit() {
             return _And<
+                __tuple_constructible<_Tuple, tuple>,
                 __tuple_convertible<_Tuple, tuple>,
                 _PreferTupleLikeConstructor<_Tuple>
             >::value;

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_move.pass.cpp

@@ -46,6 +46,20 @@ struct D
     explicit D(int i) : B(i) {}
 };
 
+struct BonkersBananas {
+  template <class T>
+  operator T() &&;
+  template <class T, class = void>
+  explicit operator T() && = delete;
+};
+
+void test_bonkers_bananas_conversion() {
+  using ReturnType = std::tuple<int, int>;
+  static_assert(std::is_convertible<BonkersBananas, ReturnType>(), "");
+  static_assert(!std::is_constructible<ReturnType, BonkersBananas>(), "");
+
+}
+
 int main(int, char**)
 {
     {