reflect: Value.Method does not do the same nil checks as the language

[dotaheor]

What version of Go are you using (go version)?

go version go1.12 linux/amd64

What did you do?

package main

import (
	"fmt"
	"reflect"
)

type S struct {
	x int
}
func (S) M(int) {}


type T struct {
	*S
}

func main() {
	var v = T{}
	
	t := reflect.TypeOf(v)
	fmt.Println(t, "has", t.NumMethod(), "methods:")
	for i := 0; i < t.NumMethod(); i++ {
		fmt.Print("  method#", i, ": ", t.Method(i).Type, "\n")
	}
	fmt.Println(reflect.ValueOf(v).Method(0)) // ok
	_ = v.M // panic
}

What did you expect to see?

Run okay.

What did you see instead?

panic.

Update: similarly for interface dynamic values

[cuonglm]

In var v = T{}, the *S embedded in T is nil, causing panic.

The program works as expected.

[ianlancetaylor]

Yes, this is a value method, but there is no value. When using Method(0), you will get a panic if you try to call the method. If anything we should change reflect.Value.Method to check for this case and panic then, rather than returning a reflect.Value that can not be used.

[dotaheor]

@cuonglm there is an inconsistency between refection and direct code.

@ianlancetaylor I think it in another way. The program prints

main.T has 1 methods:
  method#0: func(main.T, int)
...

which indicates type T has type method with the first parameter type as T.
It looks compilers declare an implicit method for both type T and *T:

func (t T) M(int) {
    t.S.M()
}

func (t *T) M(int) {
    (*t).M()
}

So I think the gc compiler does some unnecessary thing when evaluating v.M.

[dotaheor]

It looks gccgo implements it correctly. The following program prints two same values if it is compiled with gc, but two different values if it is compiled with gccgo.

package main

import (
	"fmt"
)

type S struct {
	x int
}
func (S) M(int) {}


type T struct {
	*S
}

func main() {
	var v = T{&S{}}
	fmt.Println(v.M)
	fmt.Println(v.S.M)
}

[dotaheor]

But gccgo also makes the program in the first comment panic.

[dotaheor]

A short one which indicates T and *T both has a M method.

package main

type S struct {
	x int
}
func (S) M(int) {}


type T struct {
	*S
}

func main() {
	var v = T{}

	_ = T.M    // ok
	_ = (*T).M // ok
	_ = v.M    // panic
}

[dotaheor]

An example which proves the current reflection implementation is correct.

package main

import (
	"fmt"
	"reflect"
)

type S struct {
	x int
}
func (s S) M(n int) {
	fmt.Println(s.x, n)
}


type T struct {
	*S
}

func main() {
	var v = &T{}
	
	f := reflect.ValueOf(v).Elem().Method(0)
	v.S = &S{x: 123}
	f.Call([]reflect.Value{reflect.ValueOf(789)})
}

[ianlancetaylor]

Thanks for that final example. I'm willing to believe that there is nothing to do here.

[dotaheor]

@ianlancetaylor, so do you think type T and its values haven't a M method?

@griesemer, how do you think?

[dotaheor]

I think it is no doubt that there is an inconsistency between refection and direct code, whether or not type T and its values haven't a M method.

package main

import (
	"fmt"
	"reflect"
)

type S struct {
	x int
}
func (s S) M(n int) {
	fmt.Println(s.x, n)
}


type T struct {
	*S
}

func main() {
	{
		var v = &T{}
		
		f := reflect.ValueOf(v).Elem().Method(0)
		v.S = &S{x: 123}
		f.Call([]reflect.Value{reflect.ValueOf(789)}) // 123 789
	}
	
	// If the above reflection code works,
	// the following direct one should also.
	// However, it is not.

	{
		var v = &T{}
		f := v.M // panic
		v.S = &S{x: 123}
		f(789)
	}
}

[dotaheor]

I think, whether or not type T and its values haven't a M method, the outputs of the reflection code and direct code should be always consistent. But they are not.

package main

import (
	"fmt"
	"reflect"
)

type S struct {
	x int
}
func (s S) M(n int) {
	fmt.Println(s.x, n)
}


type T struct {
	*S
}

func main() {
	{
		var v = &T{&S{123}}
		f := v.M
		f(789) // 123 789
		v.S = nil
		f(789) // 123 789
	}
	
	{
		var v = &T{&S{123}}
		f := reflect.ValueOf(v).Elem().Method(0)
		f.Call([]reflect.Value{reflect.ValueOf(789)}) // 123 789
		v.S = nil
		f.Call([]reflect.Value{reflect.ValueOf(789)}) // panic
	}
}