spec: when do struct comparisons panic?

[bradfitz]

Should the spec clarify when a struct comparison panics?

e.g.

https://play.golang.org/p/4NUu89RI27t

package main

import "fmt"

func cmp(a, b S) {
	defer func() {
		if recover() != nil {
			fmt.Println("panic")
		}
	}()
	if a == b {
		fmt.Println("equal")
	} else {
		fmt.Println("not-equal")
	}
}

type S struct{ a, b interface{} }

func main() {
	var f func()
	cmp(S{1, f}, S{2, f})
	cmp(S{f, 1}, S{f, 2})
}

Is it okay for implementations to have different answers to those two prints?

(Background: https://go-review.googlesource.com/c/go/+/230207, cmd/compile: improve generated eq algs for structs containing interfaces from @josharian)

/cc @griesemer @ianlancetaylor @mdempsky

[josharian]

I believe the relevant part of the spec is:

A comparison of two interface values with identical dynamic types causes a run-time panic if values of that type are not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields.

A very literal reading of the spec suggests Brad’s program should print “panic, panic”. Currently it prints “not-equal, panic”. The CL Brad linked to would not change the output for this particular program, but for similar programs, it might change the output to “not-equal, not-equal”.

[randall77]

It would be unfortunate if, when comparing [100]interface{}, we had to compare all 100 entries even when the first entry reports !=.
Not the end of the world, though.

[mdempsky]

I think this is #8606

[bradfitz]

@mdempsky, thanks! Closing this one as a duplicate.

[zigo101]

If both of the filed names are changed to _, then the comparisons results are equal. Bug?

package main

import "fmt"

func cmp(a, b S) {
	defer func() {
		if recover() != nil {
			fmt.Println("panic")
		}
	}()
	if a == b {
		fmt.Println("equal")
	} else {
		fmt.Println("not-equal")
	}
}

type S struct{_, _ interface{} }

func main() {
	var f func()
	cmp(S{1, f}, S{2, f}) // equal
	cmp(S{f, 1}, S{f, 2}) // equal
}

[mdempsky]

https://golang.org/ref/spec#Comparison_operators:

"Two struct values are equal if their corresponding non-blank fields are equal."