There's a proposal [0] by Ian Lance Taylor that proposes extending the type parameter constrains union of types to non-generic interfaces too. Indeed, there are

There's a proposal [0] by Ian Lance Taylor that proposes extending the type parameter constrains union of types to non-generic interfaces too. Indeed, there are some issues with the implementation details, but I'm hoping they get ironed out. [0] https://github.com/golang/go/issues/57644 https://github.com/golang/go/issues/57644