It is not inheritance, it is called supertrait. It is a bound for the trait that if someone implements your
CustomErc20, he also should implement the
ink! right now doesn't support it(your error says to you about it). Maybe it will be fixed during this issue or this.
You can use
openbrush::trait_definition macro that allows adding bounds. But you need to use it in combination with the
openbrush::contract macro(instead of
Right now, OpenBrush allows adding any bounds(defined with
ink::trait_definition and common rust traits). But in the next release, OpenBursh will allow adding only bounds defined via
openbrush::trait_definition. It is because traits defined with the
trait_definition macro are public and should not use bounds related to a specific language or implementation.