The method setLocale() does not exist on Illuminate\Container\Container. Are you sure you never get this type here, but always one of the subclasses?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
Accessing locale on the interface Illuminate\Contracts\Auth\Authenticatable suggest that you code against a concrete implementation. How about adding an instanceof check?