type_parameter_supertype_of_its_bound

'{0}' can't be a supertype of its upper bound.

The analyzer produces this diagnostic when the bound of a type parameter (the type following the extends keyword) is either directly or indirectly the type parameter itself. Stating that the type parameter must be the same as itself or a subtype of itself or a subtype of itself isn't helpful because it will always be the same as itself.

The following code produces this diagnostic because the bound of T is T:

class C<T extends T> {}

The following code produces this diagnostic because the bound of T1 is T2, and the bound of T2 is T1, effectively making the bound of T1 be T1:

class C<T1 extends T2, T2 extends T1> {}

If the type parameter needs to be a subclass of some type, then replace the bound with the required type:

class C<T extends num> {}

If the type parameter can be any type, then remove the extends clause:

class C<T> {}

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.