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I created a pull request, my approver requested some changes, i marked those resolved and clicked on re-request button with out seeing if the merging was blocked or unblocked. So now to unblock merging, do approver need to review the changes again and approve it ? Beacause the below message did not say it needs to be approved by reviewer.

Merging can be performed automatically once the requested changes are addressed.

Answers

Yes, you need to have your reviewer approve the changes. Just because you think you addressed the changes doesn't mean the reviewer agrees, and the only way GitHub can know that they've been addressed is to have the reviewer approve the changes. They might disagree and request more changes.

Part of the reason this functionality exists is to provide an effective control, which is required in many regulated industries before code can be merged or deployed. It wouldn't be very effective if the author could vouch for their own changes.