A regression test for a brittle Equal method.

[filmil]

This seems to me to demonstrate that the contract for the Equal method should
either be precisely specified, or that go-cmp should be fixed to ensure that
it behaves sensibly when encountered with nils.

[dsnet]

Anytime you indirect a pointer you should either:

• Check whether the pointer is nil
• Verify that the code leading up to that point guarantees that the pointer is set
• Or decide that a panic is exactly what you want.

For an Equal method there is a sensible definition of what should happen:

func (x *foo) Equal(y *foo) bool {
	if x == nil || y == nil {
		return x == nil && y == nil
	}
	return x.value == y.value
}

Thus, I argue that this is a bug in the definition of Equal.

Alternatively, the Equal method could be defined on the non-pointer receiver version. The time.Time type uses this pattern.

func (x foo) Equal(y foo) bool {
	return x.value == y.value
}

[dsnet]

Closing this as it seems this was demonstrate the problem with the associated issue.