Hello!

std::optional<T> has a really greedy converting constructor. It accepts
almost any type that T is constructible from, and constructs a T from
it, even if that type is also convertible to optional<T>. Example:
https://gcc.godbolt.org/z/ncsYehW19

     #include <iostream>
     #include <optional>
     #include <source_location>

     struct A
     {
         template <typename T>
         operator T()
         {
             std::cout <<
std::source_location::current().function_name() << '\n';
             return {};
         }
     };

     int main()
     {
         std::optional<int> x(A{}); // (1) Calls `operator int`, not
`operator optional<int>`.
         // std::optional<int> y = A{}; // (2) Ambiguous.
         x = A{}; // (3) Calls `operator int`, not `operator optional<int>`.
     }

This is very confusing in my eyes. std::optional is first type I see in
the wild that doesn't play nice with templated conversion operators.

I suggest we make this constructor of optional not participate in
overload resolution if the argument is convertible to optional<T>.

This would make (2) valid, and make all three lines call A::operator
optional<T>.