The ConvertibleTo concept requires an expression of a particular
type and value category to be both implicitly and explicitly convertible to some
other type.

The implicit and explicit conversions are required to produce equal
results.

```
template<class From, class To>
concept ConvertibleTo =
is_convertible_v<From, To> &&
requires(From (&f)()) {
static_cast<To>(f());
};
```

Let test be the invented function:

To test(From (&f)()) { return f(); }and let f be a function with no arguments and return type From such that f() is equality-preserving.

ConvertibleTo<From, To> is satisfied only if:

- From is not a reference-to-object type, or
- If From is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by f() after either above expression is valid but unspecified ([lib.types.movedfrom]).
- Otherwise, the object referred to by f() is not modified by either above expression.