In our case my husband arrived in Canada in the early summer 2018 but did not get a SIN till January. However since we were able to conclude that since we got married in 2018 and thus he has significant ties to Canada we decided to file taxes for him for 2018. We were able to get his tax forms from his home country and declare his income overseas as foreign income (and 0 for income earned in Canada) and it was filed and we received a notice of assessment about 2 weeks after mine arrived (we had to do a physical tax mail in as first time filings cannot be done electronically). In the SIN section I wrote his newly acquired temporary number with a letter of explanation about his status with copies of immigration letters, his foreign tax return, the exchange to Canadian dollars with our reference and it all worked out except they asked recently for his 2017 in which we got a notarized affidavit as directed in the letter we received stating he was not a resident in Canada during this period and unable to file taxes (it was mostly pertaining to MSP and Pharmacare coverage but it all links to CRA)
Also from what I understand you can write 000-000-000 for the SIN number if you do not have a number at the time you file taxes because the government expects everyone who is a resident in Canada to file taxes regardless of income earned even if it is 0
