Why is the code accepted by clang but rejected by vc++? - c++

Question

clang 3.4 accepts the following code; while vc++ NOV 2013 CTP rejects it with an error:
error C2668: 'AreEqual' : ambiguous call to overloaded function
template<class headT, class... tailTypes>
constexpr headT&& __GetFirst__(headT&& value, tailTypes&&...)
{
return static_cast<headT&&>(value);
};
template<class T>
constexpr bool AreEqual(const T& a, const T& b)
{
return a == b;
}
template<class headT, class... tailTypes>
constexpr bool AreEqual(const headT& head_value, const tailTypes&... tail_values)
{
return AreEqual(head_value, __GetFirst__(tail_values...))
&& AreEqual(tail_values...);
}
int main()
{
AreEqual(1, 1, 2, 1);
}
Which compiler is correct as per the C++14 standard?
Update: The full error message:
error C2668: 'AreEqual' : ambiguous call to overloaded function
1> d:\projects\ktl\test\main.cpp(20): could be 'bool AreEqual<headT,int>(const headT &,const int &)'
1> with
1> [
1> headT=int
1> ]
1> d:\projects\ktl\test\main.cpp(8): or 'bool AreEqual<headT>(const T &,const T &)'
1> with
1> [
1> headT=int
1> , T=int
1> ]
1> while trying to match the argument list '(const int, const int)'
1>
1>Build FAILED.

Answer 1

Clang (and GCC)'s behavior is correct. You can read §14.8.2.4 [temp.deduct.partial] of the standard for how the partial ordering for function templates is done, but the example given in p8 of that subclause directly covers this situation:
template<class... Args> void f(Args... args); // #1
template<class T1, class... Args> void f(T1 a1, Args... args); // #2
template<class T1, class T2> void f(T1 a1, T2 a2); // #3
f(); // calls #1
f(1, 2, 3); // calls #2
f(1, 2); // calls #3; non-variadic template #3 is more
// specialized than the variadic templates #1 and #2