cmp: top-level interfaces are not matched about go-cmp

[dsnet]

In working on this, I attempted to always promote to an interface{} when retrieving value. Thus,

go-cmp/cmp/compare.go

Line 87 in 5411ab9

s.compareAny(reflect.ValueOf(x), reflect.ValueOf(y))

would become something like:

iface := reflect.TypeOf((*interface{})(nil)).Elem()
s.compareAny(reflect.ValueOf(x).Convert(iface), reflect.ValueOf(y).Convert(iface))

However, this does not work because the cmp package uses assignability to determine applicability of filters and options and interface{} is not assignable to error even if the underlying concrete type may be assignable to error.

My next thought was to dynamically create an interface type T that had the common set of methods for both the x and y values. If the underlying types of x and y are each individually assignable to some interface R, then it is guaranteed that T is also assignable to R. However, this approach requires dynamically creating an interface, which is functionality that reflect does not currently have. I commented on golang/go#4146 (comment) with this use-case.

On that thread, @Merovius commented:

I might misread this, but the documentation says that

The comparer f must be a function "func(T, T) bool" and is implicitly filtered to input values assignable to T. If T is an interface, it is possible that f is called with two values of different concrete types that both implement T.

That would seem to cover the case you mention? In particular this works.

You are correct that the Comparer itself allows for "two values of different concrete types that both implement T", but this is not property assumed by the rest of the cmp. For example, the first rule on cmp.Equal is:

If two values are not of the same type, then they are never equal and the overall result is false.

This rule prevents Equal from even attempting to use filters or comparers. Note that we cannot relax this restriction as this assumption is heavily depended upon.

[dsnet]

Fixed by #112.