This:

int Foo(int a, int b)
{
    return a;
}

Becomes:

int Foo(int a)
{
    return a;
}

Don't offer a diagnostic if:

• The method is an interface implementation or an override.
• It is a constructor on a serializable object (see pattern here).
• It is an event handler (object and then an EventHandler).

This is a Warning, Category Usage.
Diagnostic id: CC0057