Why implicit non-pointer methods not satisfy interface?

Question

Assuming we have an understanding that,

For explicit method definition for type X, GO compiler implicitly defines the same method for type *X and vice versa, if I declare,

func (c Cat) foo(){
  //do stuff_
} 

and declare,

func (c *Cat) foo(){
  // do stuff_
}

then GO compiler gives error,

Compile error: method re-declared

which indicates that, pointer method is implicitly defined and vice versa

---

In the below code,

package main

type X interface{
  foo();
  bar();
}

type Cat struct{

}

func (c Cat) foo(){
  // do stuff_
}

func (c *Cat) bar(){
  // do stuff_
}

func main() {
  var c Cat
  var p *Cat
  var x X

  x = p // OK; *Cat has explicit method bar() and implicit method foo()
  x = c //compile error: Cat has explicit method foo() and implicit method bar()

}

---

GO compiler gives error,

cannot use c (type Cat) as type X in assignment:
    Cat does not implement X (bar method has pointer receiver)

at x = c, because, implicit pointer methods satisfy interfaces, but implicit non-pointer methods do not.

Question:

Why implicit non-pointer methods do not satisfy interfaces?

Answer 1

Let's look into the language specification:

A type may have a method set associated with it. The method set of an interface type is its interface. The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T).

In your example, the method set of the interface type x is [foo(), bar()]. The method set of the type Cat is [foo()], and the method set of the type *Cat is [foo()] + [bar()] = [foo(), bar()].

This explains, why variable p satisfies the interface x, but variable c doesn't.