How to determine function output type based on template parameters?

The following construct is a minimal example of what I try to achieve: let the return type of a templated function depend on the input types. First I wrote (below a selection of a set that also specializes for std::complex<> variants):

template<class U, class V> struct result_type                { using type = U; };
template<>                 struct result_type<float, double> { using type = double; };
template<>                 struct result_type<double, float> { using type = double; };

Normally you could somewhere use in code:

template<typename U, typename V>
void function()
{ 
    using R = typename result_type<U, V>::type;
    ...
}

But in my case I need the construct:

template<typename U, typename V>
R function()  // Determine 'R' by result_type<U, V>::type
{         
    ...
}

I cannot find the right code to do this. Can this be done anyway?

>Solution :

This is one way to use your type trait to define the return type of the function:

template<typename U, typename V>
typename result_type<U, V>::type function()
{         
    // return {}; // ...
}

A possible simplification could be to make use of std::common_type:

template<typename U, typename V>
std::common_type_t<U, V> function() {
    // return {}; // ...
}