LWG-3158 tuple(allocator_arg_t, const Alloc&) should be conditionally explicit (#195)

Resolves #69.

stl/inc/tuple

@@ -419,10 +419,28 @@ public:
         : tuple(_Unpack_tuple_t{}, _STD move(_Right)) {}
 #endif // ^^^ !_HAS_CONDITIONAL_EXPLICIT ^^^
 
+#if _HAS_CONDITIONAL_EXPLICIT
     template <class _Alloc, class _This2 = _This,
         enable_if_t<conjunction_v<is_default_constructible<_This2>, is_default_constructible<_Rest>...>, int> = 0>
+    explicit(
+        !conjunction_v<_Is_implicitly_default_constructible<_This2>, _Is_implicitly_default_constructible<_Rest>...>)
+        tuple(allocator_arg_t, const _Alloc& _Al)
+        : _Mybase(allocator_arg, _Al), _Myfirst(_Al, allocator_arg) {}
+#else // ^^^ _HAS_CONDITIONAL_EXPLICIT ^^^ / vvv !_HAS_CONDITIONAL_EXPLICIT vvv
+    template <class _Alloc, class _This2 = _This,
+        enable_if_t<conjunction_v<is_default_constructible<_This2>, is_default_constructible<_Rest>...,
+                        _Is_implicitly_default_constructible<_This2>, _Is_implicitly_default_constructible<_Rest>...>,
+            int> = 0>
     tuple(allocator_arg_t, const _Alloc& _Al) : _Mybase(allocator_arg, _Al), _Myfirst(_Al, allocator_arg) {}
 
+    template <class _Alloc, class _This2 = _This,
+        enable_if_t<conjunction_v<is_default_constructible<_This2>, is_default_constructible<_Rest>...,
+                        negation<conjunction<_Is_implicitly_default_constructible<_This2>,
+                            _Is_implicitly_default_constructible<_Rest>...>>>,
+            int> = 0>
+    explicit tuple(allocator_arg_t, const _Alloc& _Al) : _Mybase(allocator_arg, _Al), _Myfirst(_Al, allocator_arg) {}
+#endif // ^^^ !_HAS_CONDITIONAL_EXPLICIT ^^^
+
 #if _HAS_CONDITIONAL_EXPLICIT
     template <class _Alloc, class _This2 = _This,
         enable_if_t<_Tuple_constructible_v<tuple, const _This2&, const _Rest&...>, int> = 0>