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conflicting_generic_interfaces

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The {0} '{1}' can't implement both '{2}' and '{3}' because the type arguments are different.

Description

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The analyzer produces this diagnostic when a class attempts to implement a generic interface multiple times, and the values of the type arguments aren't the same.

Example

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The following code produces this diagnostic because C is defined to implement both I<int> (because it extends A) and I<String> (because it implementsB), but int and String aren't the same type:

dart
class I<T> {}
class A implements I<int> {}
class B implements I<String> {}
class C extends A implements B {}

Common fixes

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Rework the type hierarchy to avoid this situation. For example, you might make one or both of the inherited types generic so that C can specify the same type for both type arguments:

dart
class I<T> {}
class A<S> implements I<S> {}
class B implements I<String> {}
class C extends A<String> implements B {}

Unless stated otherwise, the documentation on this site reflects Dart 3.8.1. Page last updated on 2025-05-08. View source or report an issue.