started on custom_executor docs.

[anarthal]

No. prefer_only makes the property on any_io_executor preferable, but not requirable.

This is a guess, but it's likely that the only hard requirement is blocking.never and the context one (that's used to retrieve the execution_context pointer).

My theory is that if a property cannot be required, prefer will just return the original executor. But I'd have to test it.

[klemens-morgenstern]

Tested it, seems to be the case.

[klemens-morgenstern]

See https://gcc.godbolt.org/z/qEj98M5KT