Impossible comparison of interface value with untyped `nil`GO-W5006

Under the covers, interfaces are implemented as two elements, a type T and a value V. V is a concrete value such as an int, struct, or pointer, never an interface itself, and has type T. For instance, if we store the int value 3 in an interface, the resulting interface value has, schematically, (T=int, V=3).

The value V is also known as the interface’s dynamic value since a given interface variable might hold different values V (and corresponding types T) during the execution of the program.

An interface value is nil only if the V and T are both unset (T=nil, V is not set); in particular, a nil interface will always hold a nil type. If we store a nil pointer of type *int inside an interface value, the inner type will be *int regardless of the value of the pointer: (T=*int, V=nil). Such an interface value will therefore be non-nil even when the pointer value V inside is nil. This situation can be confusing and arises when a nil value is stored inside an interface value such as an error return:

func returnsError() error {
    var p *MyError = nil
    if bad() {
        p = ErrBad
    }
    return p // Will always return a non-nil error.
}

If all goes well, the function returns a nil p, so the return value is an error interface value holding (T=*MyError, V=nil). This means that if the caller compares the returned error to nil, it will always look as if there was an error even if nothing bad happened. To return a proper nilerror to the caller, the function must return an explicit nil:

func returnsError() error {
    if bad() {
        return ErrBad
    }
    return nil
}

It’s a good idea for functions that return errors always to use the error type in their signature (as we did above) rather than a concrete type such as *MyError, to help guarantee the error is created correctly. As an example, os.Open returns an error even though, if not nil, it’s always of concrete type *os.PathError.

Similar situations to those described here can arise whenever interfaces are used. Just keep in mind that if any concrete value has been stored in the interface, the interface will not be nil. For more information, see The Laws of Reflection (https://golang.org/doc/articles/laws_of_reflection.html).

This text has been copied from https://golang.org/doc/faq#nil_error, licensed under the Creative Commons Attribution 3.0 License.