在此示例中，为什么模板参数的顺序对MS C ++编译器重要？

Question

The following code compiles fine in GCC but in Visual Studio it results in

error C2782: ' bool contains(const T &,const std::initializer_list<T2> &) ' : template parameter ' T ' is ambiguous see declaration of ' contains ' could be ' const wchar_t * ' or ' std::wstring '

It does however compile and work if the order of the template parameters is given as

template<typename T2, typename T>

Is this a compiler bug?

#include <string>
#include <iostream>
#include <set>
#include <initializer_list>
#include <algorithm>

template<typename T, typename T2>
bool contains(T const& value, std::initializer_list<T2> const& set)
{
  return std::find(std::begin(set), std::end(set), value) != std::end(set);
}

int main(void)
{
  std::set<std::wstring> values = { L"bar", L"not" };

  for (std::wstring val : values) {
    std::wcout << "\"" << val << "\" ";
    if (contains(val, { L"foo", L"bar", L"baz", L"doom" })) {
      std::wcout << "found" << std::endl;
    }
    else {
      std::wcout << "not found" << std::endl;
    }
  }
}

Edit: I have created a bugreport: https://connect.microsoft.com/VisualStudio/feedbackdetail/view/982338/template-parameter-order-matters

Answer 1

I remember that VS has a bug where they would do double-deduction in certain scenarios, and I think that's what's happening here. Clang also compiles it both ways, so since clang + gcc agree, it's likely a VS bug.

Answer 2

I had a similar problem which was resolved by switching to the latest VS Pro version. I think this bug was addressed in the latest VS pro version as I remember seeing it in a change-log at some point.