The doc says:

> Generic types can have methods. The receiver type of a method must declare the same number of type parameters as are declared in the receiver type's definition. They are declared without the type keyword or any constraint.

So my original example should have been:

  func (obj *SomeType(K, V)) Foo(key K, val V) (*OtherType(K, V), error) {
        ...
  }

(hopefully this is now correct!).