What is the 'typed nil' trap in Go and why does 'if err != nil' sometimes fail?

An interface value in Go has two components: a dynamic type and a dynamic value. An interface is nil only when BOTH are nil. A common mistake is returning a typed nil pointer as an error — the interface has a type component set, so it is NOT nil even though the pointer value is nil.

// The trap: returning a *MyError that is nil as an error interface
type MyError struct{ Code int }
func (e *MyError) Error() string { return fmt.Sprintf("error %d", e.Code) }

func riskyOperation(succeed bool) error {
    var err *MyError   // err is a nil *MyError pointer
    if !succeed {
        err = &MyError{Code: 404}
    }
    return err         // BUG: returns interface{type=*MyError, value=nil}
                       // This interface is NOT nil!
}

result := riskyOperation(true)  // supposed to succeed
if result != nil {              // UNEXPECTED: true! Interface is non-nil.
    fmt.Println("error:", result)  // prints: error 0
}

// FIX: return bare nil, not a typed nil pointer
func safeOperation(succeed bool) error {
    if !succeed {
        return &MyError{Code: 404}
    }
    return nil   // correct: returns a nil interface, not a *MyError nil
}

// Verify the fix
result2 := safeOperation(true)
fmt.Println(result2 == nil)  // true — correct!

// Rule: functions returning an error interface should NEVER return
// a concrete typed pointer that might be nil. Always return bare nil.

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