add examples for AllowUnexported and IgnoreUnexported about go-cmp HOT 8 OPEN

Basic usage of them seems simple enough, for example:

func (e Example) Equals(e2 Example) bool {
	return cmp.Equal(e, e2, cmp.AllowUnexported(Example{}))
}

But even a bit more advanced seems a little odd to me. For example, using AllowUnexported together with IgnoreFields. It still gives a name must be exported panic for the IgnoreFields option when I try to ignore an unexported field.

Some examples for this would be nice.

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I think it may be reasonable for IgnoreUnexported to be the default behaviour only if there is a way to signal to the user that unexported fields are not checked.

When I first used cmp.Diff in replacement of reflect.DeepEqual, it didn't occur to me that unexported fields will not be checked until the tests panic-ed. If, hypothetically, IgnoreUnexported was set as the default, I would have taken the passing tests as indication that the swap was successful even though the new tests will not cover changes in unexported fields.

Additionally, I think the current behaviour provides an explicit visual indication to other readers that this comparison ignores unexported fields.

if diff := cmp.Diff(a, b, cmpopts.IgnoreUnexported(SomeStruct{})); diff != "" {
	t.Errorf("mismatch (-got +want):\n%s", diff)
}

I know that this behaviour is already documented in the package documentation but, perhaps, what may be helpful (especially to new users) are more examples on the usage of these options.

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Is there any way to IgnoreUnexported when comparing slices of a specific struct?

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IgnoreOptions also makes a confusing statement to the effect of "never use this for types you don't control" because of some hand-wavy statement about implementation details changing in the future.

Perhaps a controversial view: IgnoreUnexported should be the default for all types. You cannot check unexported fields anyway, and even if you could, doing so in test would violate the basic tenet of Go readability, which is to test public behavior.

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Hey there ! any update on this one ? I'm actually interested too ! :p

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Feel free to mail out a PR fixing this.

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Can you give an example?

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it's not easy to understand in my first impression
eg.
GetIssueAdvancedReply_Data is member of GetIssueAdvancedReply, GetIssueAdvancedReply_Data contains some unexported fields. if want to ingore it, it seems should be:

cmp.Equal(tt.want, tt.args.rsp, cmpopts.IgnoreUnexported(pb.GetIssueAdvancedReply{}, pb.GetIssueAdvancedReply_Data{})

hope help

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