None of the languages you mentioned support this 'eager_fallback' feature, no. I am not stuck on any one application of the feature (you'll notice I referred both to the individual application and the general feature, and then you ignored it), but rather you are overindexing on generic returns, which are insufficient for what you're describing in a load-bearing way. Rust supports what C# supports, on the subject of generics (in ways germane to this feature), and what Rust lacks, C# also lacks. So your reference to C# supporting this is false. The feature you are proposing exists in zero languages on the planet. The thing you can get, top-generic returns, exist in Rust and the languages you mentioned, and don't accomplish your named goal at all because you can't use them as a method receiver without disambiguating them.