extension_type_implements_representation_not_supertype

'{0}', the representation type of '{1}', is not a supertype of '{2}', the representation type of '{3}'.

The analyzer produces this diagnostic when an extension type implements another extension type, and the representation type of the implemented extension type isn't a subtype of the representation type of the implementing extension type.

The following code produces this diagnostic because the extension type B implements A, but the representation type of A (num) isn't a subtype of the representation type of B (String):

extension type A(num i) {}

extension type B(String s) implements A {}

Either change the representation types of the two extension types so that the representation type of the implemented type is a supertype of the representation type of the implementing type:

extension type A(num i) {}

extension type B(int n) implements A {}

Or remove the implemented type from the implements clause:

extension type A(num i) {}

extension type B(String s) {}