Why Does `std::function`'s Template Argument Lead to Ambiguity in Overload Resolution?

Ambiguity Residing in Std::Function's Template Argument Despite Defining Type

In this scenario, multiple overloads exist for function 'a', each accepting a distinct signature of std::function:

• function
• function

The ambiguity arises due to the fact that both function and function are constructible from the same function. The std::function constructor, as defined in VS2010, allows for construction from diverse sources:

template<class _Fx>
function(_Fx _Func, typename _Not_integral<!...int>::_Type = 0);

Essentially, any object can be converted into a std::function, facilitating type erasure and enabling the handling of arbitrary objects as functions.

To further illustrate the problem, consider the following simplified example:

template<class Signature>
class myfunc{
   public:
      template<class Func>
      myfunc(Func a_func){
          // ...
      }
};

During overload resolution for 'a', if a viable overload cannot be identified, the compiler attempts to convert the arguments using either the parameter's constructor or the argument's conversion operator. In this case, the conversion occurs via the constructor of myfunc.

Since myfunc's constructor accepts anything, both function and function can be converted to myfunc.

Ultimately, this results in the compiler being unable to determine which overload of 'a' to invoke, leading to the ambiguity.

To resolve this issue, you have several options:

1. Maintain Explicit Casts:
   An explicit cast can specify the desired overload for the compiler: a((functionx))
2. Function Objects:
   Create function objects of the appropriate type: `function fx = x;
   function fy = y;`
3. Template Metaprogramming:
   Employ template metaprogramming to obtain the correct signature: `template
   struct get_signature{...};`

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