I believe tuple is still under development in the standard. Daniel Krugler is/will be making convincing arguments that a modified form of LWG 2051 (currently NAD Future) is easily acheivable and desirable. He has demonstrated that a tuple<T...> where all of the T are implicitly convertible from U... should have a tuple constructor that is also implicit, instead of explicit. This would support the use cases in LWG 2051 while not undermining T... with explicit conversions from U.... This check-in is an experimental implementation of Daniel's work. I believe this work to be mature enough to warrant inclusion into libc++. If anyone sees real-world problems that this check in causes, please let me know and I will revert it, and provide the feedback to the LWG.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@153855 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/tuple/tuple.tuple/tuple.cnstr/UTypes.fail.cpp

@@ -14,14 +14,36 @@
 // template <class... UTypes>
 //   explicit tuple(UTypes&&... u);
 
+/*
+    This is testing an extension whereby only Types having an explicit conversion
+    from UTypes are bound by the explicit tuple constructor.
+*/
+
 #include <tuple>
 #include <cassert>
 
-#include "../MoveOnly.h"
+class MoveOnly
+{
+    MoveOnly(const MoveOnly&);
+    MoveOnly& operator=(const MoveOnly&);
+
+    int data_;
+public:
+    explicit MoveOnly(int data = 1) : data_(data) {}
+    MoveOnly(MoveOnly&& x)
+        : data_(x.data_) {x.data_ = 0;}
+    MoveOnly& operator=(MoveOnly&& x)
+        {data_ = x.data_; x.data_ = 0; return *this;}
+
+    int get() const {return data_;}
+
+    bool operator==(const MoveOnly& x) const {return data_ == x.data_;}
+    bool operator< (const MoveOnly& x) const {return data_ <  x.data_;}
+};
 
 int main()
 {
     {
-        std::tuple<MoveOnly> t = MoveOnly(0);
+        std::tuple<MoveOnly> t = 1;
     }
 }

test/utilities/tuple/tuple.tuple/tuple.cnstr/const_Types.fail.cpp

@@ -20,7 +20,6 @@
 int main()
 {
     {
-        std::tuple<int> t = 2;
-        assert(std::get<0>(t) == 2);
+        std::tuple<int*> t = 0;
     }
 }