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I got bit by this the other day when returning a nil pointer to a struct as an error. It was incredibly frustrating when I finally figured out what was going on, and I'm curious if there's a good reason to distinguish between these 2 types of nil or if it was just an oversight/mistake during the language design process.

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It's not two types of nil.

When a nil value is converted to an interface, it carries type information. When an interface carries type information, it isn't nil.

Most likely the reason is keeping the language implementation simple. Java-style `null` is actually quite subtle: http://stackoverflow.com/a/2707333/46571

Go has the same complexity. From the Go spec: "For an expression x of interface type and a type T, the primary expression x.(T) asserts that x is not nil and that the value stored in x is of type T.'"

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