Continuining from yesterday, when we started disecting this section of the spec:

Method declarations

…

A receiver base type cannot be a pointer or interface type and it must be defined in the same package as the method.

Today we’re tackling the second limitation:

A receiver base type cannot be an interface.

At first this might seem a bit counter-intuitive… aren’t interfaces all about methods? Well, yes, but in the other directions. Interfaces are collections of methods. But you can’t define methods on interfaces. To be explicit, this is invalid:

type foo interface {
	Foo() error
}

func (f foo) Bar() {} // invalid receiver type foo (pointer or interface type)

If you want to extend the foo interface with the Bar() method, the proper way is to extend the foo type definition:

type foo interface {
	Foo() error
	Bar()
}

Now it’s up to any types that implement this interface to define a method Bar().

Quotes from The Go Programming Language Specification Version of August 2, 2023