extension_type_implements_not_supertype

'{0}' is not a supertype of '{1}', the representation type.

The analyzer produces this diagnostic when an extension type implements a type that isn't a supertype of the representation type.

The following code produces this diagnostic because the extension type A implements String, but String isn't a supertype of the representation type int:

extension type A(int i) implements String {}

If the representation type is correct, then remove or replace the type in the implements clause:

extension type A(int i) {}

If the representation type isn't correct, then replace it with the correct type:

extension type A(String s) implements String {}