Normalization of method selectors

Go allows simplified forms of some selectors.

For example, in the following program, t1.M1 is a simplified form of (*t1).M1, and t2.M2 is a simplified form of (&t2).M2. At compile time, the compiler will normalize the simplified forms to their original respective full forms.

The following program prints 0 and 9, because the modification to t1.X has no effects on the evaluation result of *t1 during evaluating (*t1).M1.

package main

type T struct {
	X int
}

func (t T) M1() int {
	return t.X
}

func (t *T) M2() int {
	return t.X
}

func main() {
	var t1 = new(T)
	var f1 = t1.M1 // <=> (*t1).M1
	t1.X = 9
	println(f1()) // 0 
	
	var t2 T
	var f2 = t2.M2 // <=> (&t2).M2
	t2.X = 9
	println(f2()) // 9
}

In the following code, the function foo runs okay, but the function bar will produce a panic. The reason is s.M is a simplified form of (*s.T).M. At compile time, the compiler will normalize the simplified form to it original full form. At runtime, if s.T is nil, then the evaluation of *s.T will cause a panic. The two modifications to s.T have no effects on the evaluation result of *s.T.

package main

type T struct {
	X int
}

func (t T) M() int {
	return t.X
}

type S struct {
	*T
}

func foo() {
	var s = S{T: new(T)}
	var f = s.M // <=> (*s.T).M
	s.T = nil
	f()
}

func bar() {
	var s S
	var f = s.M // panic
	s.T = new(T)
	f()
}

func main() {
	foo()
	bar()
}

Please note that, interface method values and method values got through reflection will be expanded to the promoted method values with a delay. For example, in the following program, the modification to s.T.X has effects on the return results of the method values got through reflection and interface ways.

package main

import "reflect"

type T struct {
	X int
}

func (t T) M() int {
	return t.X
}

type S struct {
	*T
}

func main() {
	var s = S{T: new(T)}
	var f = s.M // <=> (*s.T).M
	var g = reflect.ValueOf(&s).Elem().
		MethodByName("M").
		Interface().(func() int)
	var h = interface{M() int}(s).M
	s.T.X = 3
	println( f() ) // 0
	println( g() ) // 3
	println( h() ) // 3
}

Source: https://github.com/golang/go/issues/47863

Note, there was a bug in the official standard Go compiler before version 1.20. The older compilers will de-virtualize some interface methods at compile time but the de-virtualizations are made too far to be correct. For example, the following program should print 2 2, but it prints 1 2 if it is complied with the official standard Go compiler v1.19.

package main

type I interface{ M() }

type T struct{
	x int
}

func (t T) M() {
	println(t.x)
}

func main() {
	var t = &T{x: 1}
	var i I = t
	
	var f = i.M
	defer f() // 2 (correct)

	// i.M is de-virtualized as (*t).M at compile time (bug).
	defer i.M() // 1 (wrong)
	
	t.x = 2
}

The bug has been fixed in Go toolchain 1.20.

Source: https://github.com/golang/go/issues/52072