xref #29129 which implements ndarray.astype(newdtype, casting='same_value') but only for built-in NumPy numeric types.

In order to make the same_value casting work for user-defined dtypes, we need to think about

• registering a casting function which currently has no interface to say "use this casting function only for a specific casting kwarg"
• registering coercion rules which also does not allow any conditional rule depending on the cast kwarg.

Also, it seems same_value casting does not fit neatly into the post-NEP-50 casting rules we currently have, since it explicitly excludes a value-based answer to can_cast.

@seberg, I tried to summarize the essence of the problem solved by mattip#94. Did I miss or misunderstand something?