[concept.convertible]

Given types From and To and an expression E whose type and value category are the same as those of declval<From>(), convertible_to<From, To> requires E to be both implicitly and explicitly convertible to type To.

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

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template<class From, class To>
  concept convertible_to =
    is_convertible_v<From, To> &&
    requires {
      static_cast<To>(declval<From>();
    };

Let FromR be add_rvalue_reference_t<From> and test be the invented function: To test(FromR (&f)() { return f(); } and let f be a function with no arguments and return type FromR such that f() is equality-preserving.

Types From and To model convertible_to<From, To> only if:

(2.1)
To is not an object or reference-to-object type, or static_cast<To>(f() is equal to test(f).
(2.2)
FromR is not a reference-to-object type, or
- (2.2.1)
  If FromR 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]).
- (2.2.2)
  Otherwise, the object referred to by f() is not modified by either above expression.