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class_used_as_mixin

The class '{0}' can't be used as a mixin because it's neither a mixin class nor a mixin.

Description

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The analyzer produces this diagnostic when a class that is neither a mixin class nor a mixin is used in a with clause.

Example

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The following code produces this diagnostic because the class M is being used as a mixin, but it isn't defined as a mixin class:

dart
class M {}
class C with M {}

Common fixes

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If the class can be a pure mixin, then change class to mixin:

dart
mixin M {}
class C with M {}

If the class needs to be both a class and a mixin, then add mixin:

dart
mixin class M {}
class C with M {}