> Other than duck-typed languages (and I count Go as basically that), which languages actually provide this feature?

There are only like 3 significant languages with trait-based generics, and both the other ones have some way of providing orphan instances (Haskell by requiring a flag, Scala by not having a coherence requirement at all and relying on you getting it right, which turns out to work out pretty well in practice).

More generally it's an extremely common problem to have in a mature language; if you don't have a proper fix for it then you tend to end up with awful hacks instead. Consider e.g. https://www.joda.org/joda-time-hibernate/ and https://github.com/FasterXML/jackson-datatype-joda , and note how they have to be essentially first party modules, and they have to use reflection-based runtime registries with all the associated problems. And I think that these issues significantly increased the pressure to import joda-time into the JVM system library, which ultimately came with significant downsides and costs, and in a "systems" language that aims to have a lean runtime this would be even worse.