Why don't they at least allow "if err" rather than "if err != nil", i.e. allow error values to be truth-tested? I don't think I've seen that suggested anywhere

Why don't they at least allow "if err" rather than "if err != nil", i.e. allow error values to be truth-tested? I don't think I've seen that suggested anywhere, which is surprising to me. I guess it's fear of scope creep, of allowing syntactic shortcuts that might be ambiguous or dangerous in other circumstances? But I would have thought it could be restricted to error types.