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The documentation and implementation for `inspect.isbuiltin` and related functions do not match. While #4968 attempted to clarify things to make the documentation match the behavior, the actual committed changes are still wrong.
`isbuiltin` says "Return true if the object is a built-in function or a bound built-in method", but it returns false for bound built-in special methods (e.g., `[].__add__`). That's because it's implemented as `isinstance(types.BuiltinFunctionType)`, but bound special methods are a different type (`method-wrapper`). Terry Reedy's suggested fix from #4968 ("... or a bound built-in non-special method") would fix this, but the committed change didn't.
The docstring has a further problem; it says that anything that passes will have a `__self__` with "instance to which a method is bound, or None", but for functions, the `__self__` is usually the module the `PyMethodDef` comes from, not `None`. (For example, `sum.__self__` is the`'builtins` module.)
`isroutine` says "Return true if the object is a user-defined or built-in function or method." (The docstring instead says "... any kind of function or method.") But it returns false for built-in bound special methods (`[].__add__`). That's because it's implemented as `isbuiltin` or `isfunction` or `ismethod` or `ismethoddescriptor`; while `isbuiltin` picks up functions and bound non-special methods, and `ismethoddescriptor` picks up unbound special and non-special methods, nothing picks up bound special methods.
A doc-only fix for this, along with Terry's suggested fix for `isbuiltin`, would be "Return true if the object is a user-defined function or method, or a built-in function, unbound method, or bound non-special method." That sounds horrible, but that's because it actually is a horrible test in the first place.
A better fix would be to make `isbuiltin` handle both kinds of bound method. Then `isroutine` is correct as written, and both are documented correctly. (Note that there is no type in `types` corresponding to `method-wrapper`, so either `types` or `inspect` would have to add something like `BuiltinSpecialMethodType = type([].__add__)`.)
It would be even better to fix this at the source and clean up the types of builtin functions and methods, but that boat sailed with 3.0, so...