Why is the code accepted by clang but rejected by vc++?
c++, c++11, clang, compiler-errors, visual-c++
Solution
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
Problem
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. ```