non_covariant_type_parameter_position_in_representation_type

An extension type parameter can't be used in a non-covariant position of its representation type.

The analyzer produces this diagnostic when a type parameter of an extension type is used in a non-covariant position in the representation type of that extension type.

The following code produces this diagnostic because the type parameter T is used as a parameter type in the function type void Function(T), and parameters are not covariant:

extension type A<T>(void Function(T) f) {}

Remove the use of the type parameter:

extension type A(void Function(String) f) {}

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