Return nil or custom error in Go

Don't use DetailedError as a return type, always use error:

func foo(answer, x, y int) (int, error) {
    if answer == 42 {
        return 100, nil  //!! cannot use nil as type DetailedError in return argument
    }
    return 0, DetailedError{x: x, y: y}
}

The fact that your DetailedError type satisfies the error interface is sufficient to make this work. Then, in your caller, if you care about the extra fields, use a type assertion:

value, err := foo(...)
if err != nil {
    if detailedErr, ok := err.(DetailedError); ok {
        // Do something with the detailed error values
    } else {
        // It's some other error type, behave accordingly
    }
}

Reasons for not returning DetailedError:

It may not appear to matter right now, but in the future your code may expand to include additional error checks:

func foo(answer, x, y int) (int, error) {
    cache, err := fetchFromCache(answer, x, y)
    if err != nil {
        return 0, fmt.Errorf("Failed to read cache: %s", err)
    }
    // ... 
}

The additional error types won't be of DetailedError type, so you then must return error.

Further, your method may be used by other callers that don't know or care about the DetailedError type:

func fooWrapper(answer, x, y int) (int, error) {
    // Do something before calling foo
    result, err := foo(answer, x, y)
    if err != nil {
        return 0, err
    }
    // Do something after calling foo
    return result, nil
}

It's unreasonable to expect every caller of a function to understand a custom error type--this is precisely why interfaces, and in particular the error interface, exists in Go.

Take advantage of this, don't circumvent it.

Even if your code never changes, having a new custom error type for every function or use-case is unsustainable, and makes your code unreadable and impossible to reason about.

DetailedError struct zero value isn't nil but DetailedError{}. You can either return error interface instead of DetailedError

func foo(answer, x, y int) (int, error) {

or use pointer

func foo(answer, x, y int) (int, *DetailedError) {
...
//and
func (e *DetailedError) Error() string {