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docs: clarify Equal method is skipped when either value is nil #397

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docs: clarify Equal method is skipped when either value is nil #397

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6 changes: 4 additions & 2 deletions cmp/compare.go
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Original file line number Diff line number Diff line change
Expand Up @@ -58,8 +58,10 @@ import (
//
// - 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) even if x or y is nil. Otherwise, no such method exists and
// evaluation proceeds to the next rule.
// x.Equal(y) when both values are non-nil. If either value is nil, Equal
// reports whether both are nil without calling the method (so a nil
// receiver never panics). Otherwise, no such method exists and evaluation
// proceeds to the next rule.

@dsnet dsnet Jul 30, 2026

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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

//
// - Lastly, try to compare x and y based on their basic kinds.
// Simple kinds like booleans, integers, floats, complex numbers, strings,
Expand Down