I suspect we're done making changes to DeepEqual, but I'll keep this bug open for us to decide later, during the Go 1.8 dev cycle.

This seems like a legit, albeit niche, inconsistency to me. If Test's declaration is changed from type Test *Test to type Test struct { p *Test } or type Test [1]*Test and makeTest is updated accordingly, then DeepEqual returns immediately. There's nothing in DeepEqual's godocs to suggest this inconsistency is / should be present.

The obvious fix is to simply relax the deepEqualValue's "hard" predicate to include Ptr-Kinded Values. Downside is this would probably grow the number of entries needed in the visited map for typical programs that don't create cycles using only pointer types.

I suspect we can fix it somehow without regressing typical programs though.

@gavv How did you discover this problem? Do you have a real-world application that's negatively affected by it?

The obvious fix is to simply relax the deepEqualValue's "hard" predicate to include Ptr-Kinded Values.

Yes, this does work, just checked.

How did you discover this problem? Do you have a real-world application that's negatively affected by it?

It was a failing test for deepcopy module found on github. Not a real-world application though, but I assume this test worked before so it's probably a regression.

Yes, this is a bug, since type Test struct { X *Test } works fine. It's a bug that's been there since the initial Go release, but still a bug.

Another failing case:

package main

import "reflect"

type Test interface{}

func makeTest() Test {
    var t1, t2 Test
    t1 = new(Test)
    t2 = &t1
    *t1.(*Test) = &t2
    return t1
}

func main() {
    a := makeTest()
    b := makeTest()
    reflect.DeepEqual(a, b)
}

In the implementation, the goal of the 'hard' predicate is to avoid making visited too big. I think it should work to change hard to be

hard := func(t Type) bool {
    switch t.Kind() {
    case Array, Map, Slice, Struct:
        return true
    case Ptr:
        t = t.Elem()
        return t.Kind() == Ptr || t.Kind() == Interface
    }
    return false
}

CL https://golang.org/cl/31588 mentions this issue.