There was a proposal https://github.com/golang/go/issues/7512 https://github.com/golang/go/issues/7512 to make interface method return types covariant, but it d

There was a proposal https://github.com/golang/go/issues/7512 https://github.com/golang/go/issues/7512 to make interface method return types covariant, but it didn't go anywhere, and people seemed to favor the broken behavior (returning a concrete type can fail to satisfy an interface, because the crude matching doesn't care that the type has all the expected methods).