It's entirely up to you to deduct some, all or none of your 2018 RRSP contribution, however you must report the actual contribution to CRA, as it affects your lifetime limits for deductions.
Any amount of undeducted contribution for 2018 will be included in your 2018 Notice of Assessment in the total for undeducted amounts of previous years. Clearly this is a number that you need to know.
Any 2018 contribution and deduction should not exceed your 2018 deduction limit, which is shown on your 2017 Notice of Assessment.
If you owe HBP or LLP repayments, these amounts are taken off your 2018 contribution in terms of assessing your deduction limit for 2019.
