Get index by type in std::variant

Question

Is there a utility in the standard library to get the index of a given type in std::variant? Or should I make one for myself? That is, I want

Answer 1

We could take advantage of the fact that index() almost already does the right thing.

We can't arbitrarily create instances of various types - we wouldn't know how to do it, and arbitrary types might not be literal types. But we can create instances of specific types that we know about:

template  struct tag { }; // <== this one IS literal

template 
struct get_index;

template  
struct get_index>
    : std::integral_constant...>(tag()).index()>
{ };

That is, to find the index of B in variant we construct a variant, tag, tag> with a tag and find its index.

This only works with distinct types.