Nil Interface vs Nil Concrete Type

Why This Happens

Go interfaces have two components: a type descriptor and a value. An interface is nil only when both are nil. If you return a nil pointer of a concrete type, the interface has a non-nil type descriptor, so it is not equal to nil. This is one of the most confusing aspects of Go.

The Problem

type MyError struct {
    Message string
}

func (e *MyError) Error() string { return e.Message }

func validate() error {
    var err *MyError = nil
    return err // returns non-nil interface!
}

func main() {
    if err := validate(); err != nil {
        fmt.Println("got error:", err) // this executes!
    }
}

The Fix

func validate() error {
    // return nil directly, not a typed nil pointer
    return nil
}

func main() {
    if err := validate(); err != nil {
        fmt.Println("got error:", err)
    }
}

Step-by-Step Fix

1. 1

   Identify the typed nil return

   Find functions returning an interface type where a nil pointer of a concrete type is returned.

2. 2

   Return nil directly

   Replace return nilPointer with return nil to ensure the interface itself is nil.

3. 3

   Use reflect for debugging

   Use reflect.ValueOf(err).IsNil() if you need to check if the underlying value is nil.