go: cmd/compile: inconsistent behaviors in judging whether or not two types are identical

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10.1 linux/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

package m3

import "time"

type t = time.Time
type T = time.Time

type A = struct{t}
type B = struct{T}

import "m3"
import "time"

type t = time.Time
type T = time.Time

type A = struct{t}
type B = struct{T}

func main() {
   var a, b interface{} = m3.A{}, m3.B{}
   var x, y interface{} = A{}, B{}
   println(a == x) // true
   println(b == y) // true
   
   _ = A(m3.A{}) // cannot convert struct { time.Time } literal (type struct { time.Time }) to type struct { time.Time }
   _ = B(m3.B{}) // ok
   
   println(a == b) // true
   println(x == y) // true
   _ = B(A{})       // cannot convert struct { time.Time } literal (type struct { time.Time }) to type struct { time.Time }
   _ = m3.A(m3.B{}) // cannot convert struct { time.Time } literal (type struct { time.Time }) to type struct { time.Time }
}

What did you expect to see?

No errors, [edit: or all println calls, except the second, print false].

What did you see instead?

./main.go:18:9: cannot convert struct { time.Time } literal (type struct { time.Time }) to type struct { time.Time }
./main.go:23:9: cannot convert struct { time.Time } literal (type struct { time.Time }) to type struct { time.Time }
./main.go:24:12: cannot convert struct { time.Time } literal (type struct { time.Time }) to type struct { time.Time }

About this issue

Original URL
State: closed
Created 6 years ago
Comments: 27 (18 by maintainers)

Most upvoted comments

Two struct types with differently named (corresponding) embedded fields are different types, even if the embedded field types are identical. So the assignments a = b are expected to be reported as invalid at compile time.
Because of 1), runtime interface comparison of a and b values should return false, but the issue is that (basically) the same printing code that’s used in the error message (and which prints both types as struct{ int }) is used to identify those types’ symbols, so the type information is incorrect.

griesemer on Apr 9, 2018

If the error messages are correct, the problem is bigger:

package main

import (
	"fmt"
	"time"
)

type t = time.Time
type T = time.Time

type A = struct{ t }
type B = struct{ T }

func main() {
	var a A
	var b B
	fmt.Println(interface{}(b) == interface{}(a)) // true
	//fmt.Println(b == a)
}

dotaheor on Apr 7, 2018