docs: clarify Equal method is skipped when either value is nil

[Solaris-star]

Summary

Package docs for Equal said the method is used even if x or y is nil.

Actual behavior: comparePtr / compareInterface report nil equality first and return, so tryMethod never runs when either side is nil. Non-nil values still call Equal as documented.

This matches issue #363 and avoids implying a nil receiver would be invoked (and potentially panic).

Test plan

• Manual repro: nil vs non-nil *T with value-receiver Equal does not print / call method
• go test ./cmp/

Fixes #363

[dsnet]

Thanks for the PR, but I don't think this is quite correct.

The difference of whether Equal is called on nil is whether it is declared on a value receiver or not. Perhaps this is more clear:

If the values have an Equal method of the form "(T) Equal(T) bool" or
"(T) Equal(I) bool" where T is assignable to I, then use the result of
x.Equal(y). The method is called even if x or y are nil and Equal
is declared on a pointer receiver. Otherwise, no such method exists and
evaluation proceeds to the next rule.

See https://go.dev/play/p/Jf2jQPmgnZx

[Solaris-star]

Thanks for the correction and the playground example. I updated the package docs in commit c1d217f to use the suggested distinction: an Equal method on a pointer receiver is called even when either value is nil; the value-receiver case does not imply a callable nil receiver.

go test ./cmp/... passes. Could you please re-review?