Comparison of std::optional<not_null<T>> and not_null<T> is ambiguous

[MatthewNielsen27]

The comparison of std::optional<not_null<T>> and not_null<T> is ambiguous. This is possibly because they both have overloads of the equality operator on their "nullable" types, which are of equal precedence.

Here's a minimal example to repro: https://godbolt.org/z/xnEfP1acb

It should be noted that Microsoft/GSL doesn't have this issue: https://godbolt.org/z/Ed7qG5EE8

[mbeutel]

Thanks for the report.

I have yet to come up with a convincing approach to solve this problem; suggestions are welcome. We might be tempted to follow MS-GSL and only define comparison operators for two not_null<> arguments (link) rather than the set of mixed-argument overloads we have now (link); but the simpler approach is fraught with other problems: for example, comparing std::shared_ptr<T> and not_null<std::shared_ptr<T>> then fails, whereas it works fine with gsl-lite. (Repro for MS-GSL, gsl-lite.)

As an aside, isn't optional<not_null<T>> just a contrived semantic equivalent of T?

[MatthewNielsen27]

As an aside, isn't optional<not_null> just a contrived semantic equivalent of T?

Yeah, kinda. But when designing generic types / interfaces with value semantics, this issue adds some mental overhead. The pattern is used frequently in Rust, with Option<Box<T>> which is equivalently optional<not_null<T*>>.