I've lived in Sweden and Iceland, both of which have pre-filled tax returns to be submitted either on paper or electronically. Sweden is particularly easy, all I have to do is to verify the numbers and then send an SMS with a personal code printed on my return. If I need to change anything, I log into the tax office website, make the changes and submit.

I have deductions to offset paid interest. If I hire a person to do some renovation in my home, the tax office pays half of the fee directly to them and I get a deduction as well. I sold my flat this year and will need to pay tax from the winnings, deducting costs. Almost all of this will be pre-filled.

In Iceland I had a slightly more complex tax situation. I filed jointly with my ex wife. Mortages, minimum wage deduction (i.e. you don't pay tax of the amount that is the legal minimum wage), property tax (house, cars), stocks and bonds, bank deposits and earned interest, deductions for rent compensation, was all prefilled.

Compared to this, filing my dead-simple US taxes with one deduction, filling in a two page calculation worksheet, feels like the middle ages.

No matter the complexity of the tax code, there are systems in place for all of them to evaluate and error-check tax declarations. The information is most likely all there already. All you need to have is a way to authenticate tax payers, and a front end.

And if I have a reason to think the tax authorities will not include items that lower my dues, I can always hire an accountant to file for me. I simply give them my "third-party filer access code" and sign the final return.