We’ll cover the last 3 rules of selectors in one go today, since they’re all about illegal selectors.

Selectors

…

4. In all other cases, x.f is illegal.

Simple enough. The cases mentioned in rules 1-3 ar ethe only valid uses of x.f. All others are illegal.

var x int

x.chicken // illegal!

5. If x is of pointer type and has the value nil and x.f denotes a struct field, assigning to or evaluating x.f causes a run-time panic.

var x *Person       // x is of type *Person, but has the default value of `nil`
fmt.Println(x.Name) // panic: runtime error: invalid memory address or nil pointer dereference

6. If x is of interface type and has the value nil, calling or evaluating the method x.f causes a run-time panic.

var x interface { // is an interface type, but has the default value of `nil`
  Foo()
}

x.Foo() // panic: runtime error: invalid memory address or nil pointer dereference

The spec then offers its own examples, which I won’t go over, as I’ve already covered most these cases (and more) with my own commentary.

For example, given the declarations:

type T0 struct {
	x int
}

func (*T0) M0()

type T1 struct {
	y int
}

func (T1) M1()

type T2 struct {
	z int
	T1
	*T0
}

func (*T2) M2()

type Q *T2

var t T2     // with t.T0 != nil
var p *T2    // with p != nil and (*p).T0 != nil
var q Q = p

one may write:

t.z          // t.z
t.y          // t.T1.y
t.x          // (*t.T0).x

p.z          // (*p).z
p.y          // (*p).T1.y
p.x          // (*(*p).T0).x

q.x          // (*(*q).T0).x        (*q).x is a valid field selector

p.M0()       // ((*p).T0).M0()      M0 expects *T0 receiver
p.M1()       // ((*p).T1).M1()      M1 expects T1 receiver
p.M2()       // p.M2()              M2 expects *T2 receiver
t.M2()       // (&t).M2()           M2 expects *T2 receiver, see section on Calls

but the following is invalid:

q.M0()       // (*q).M0 is valid but not a field selector

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