invalid_deprecated_extend_annotation

The annotation '@Deprecated.extend' can only be applied to extendable classes.

The analyzer produces this diagnostic when the @Deprecated.extend annotation is applied to a declaration that isn't an extendable class. An extendable class is one that isn't declared with the interface, final, or sealed keywords and has at least one public, generative constructor.

The following code produces this diagnostic because the annotation is on a sealed class:

@Deprecated.extend()
sealed class C {}

Remove the annotation:

sealed class C {}