Compilation on xCode4

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My existing project and the also Demo both come up with Semantic Issues in xCode 4 such as these

warning: incompatible pointer types assigning to 'NSObject *__weak' from 'NSObject *'
warning: 'MGTwitterStatusesParser' may not respond to 'parser:didEndElement:namespaceURI:qualifiedName:'

The project works fine on xCode3.

Is there a compatibility issue?

Many thanks,

Chris.