`UndefinedMethodError` perhaps? that way we can restore the distinction between method declaration (which assigns a name, within some namespace) and method definition/implementation which gives the function its ability.
linguistically speaking, no method means the name isn’t even known, which isn’t the case here. the name exists and is known (after a successful declaration). what’s missing is the actual definition. if we lean heavily into language, perhaps only undefined method will do.
