Change https://golang.org/cl/230121 mentions this issue: cmd/compile: fix literal value can't be used in struct blank field of type array

The error message looks wrong but it's not obvious to me that the program should work. Is a composite literal allowed to set the value of a _ field?

We allow initializing blank, non-array-typed fields in non-keyed composite literals, so we should probably allow blank, array-typed fields too.

OK, SGTM.

It looks struct literals also have this problem, but slice literals not.
And not all such array/struct literals have this problem.
It looks this problem only happens in binary operator operations.

package main


type T = struct{y int} // struct literals also fail to compile
//type T = [1]int
//type T = []int // slice literals are ok
type S struct{x int; _ T}

var x, y = S{1, T{2}}, S{1, T{2}} // ok

func main() {
	println(S{1, T{2}}.x == S{1, T{3}}.x) // cannot use _ as value
}

Does it also only happen in structs with <= 4 fields? If so, the bug might be in the inlined struct comparison code in walkCompare in walk.go.

Does it also only happen in structs with <= 4 fields? If so, the bug might be in the inlined struct comparison code in walkCompare in walk.go.

Seems that involves alias type, using struct directly compiling ok.

Does it also only happen in structs with <= 4 fields? If so, the bug might be in the inlined struct comparison code in walkCompare in walk.go.

Seems that involves alias type, using struct directly compiling ok.

Ah no, so it works with 0-field struct.

package main

import "fmt"

func cmpS(a, b S) {
	defer func() {
		if recover() != nil {
			fmt.Println("panic")
		}
	}()
	fmt.Println(a == b)
}

type T struct{}
type S struct{x int; _ T}

func main() {
	cmpS(S{1, T{}}, S{1, T{}}) // compile ok
}

It's also ok with [0]int.