Brilliant yellow flower - is it a weed?

This brilliant yellow flower has grown up through a lilac tree that has the bottom of the trunk covered in ivy. The leaves are smooth and shiny (see V) almost like holly leaves without prickles. Is it a weed of some sort?

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Hi, looks like Mahonia aquifolium, not a weed, quite nice shrubs..

 

Thank you Bilbo.
It's very interesting finding all these new plants when
you move to a new home but I don't recall seeing it last
year. I suppose they are perennial plants.

 

It is a shrub M.aquifolium but unlike most of the other Mahonias this one does sucker freely, so if its cut to the ground it will probably shoot back up from the roots..

 

you will also find that it self-seeds fairly freely. Can be useful, but also a little annoying. I leave them to finish flowering, then whip them out!