non-compulsory foreign taxes

My guess is that the foreign tax credit is a credit for INCOME tax and contributions you are referring to is not an income tax -- compulsory or not. In USA, we don't get to a tax credit or deductions for our Social Security ("SS") contributions.

I think in some treaties, if you contribute to your own country's equivalent of our SS, you may be exempt from making the Social Security contributions on US Earnings. If that is the case, you also would not be entitled to SS benefits in your old age.

Here is a comparable situation with a different tax - California SDI. Some employees are subject to it, and some others are not. Those that are not may opt in and pay it if they want to be covered by the benefit of it. For those that are under the mandate to pay it, it is a tax deductible on Schedule A. For those that did not fall under the mandate but opted in, even though they pay the same tax in the same amount that they'd've paid had they fell under the mandate, there is NO deduction, specifically because it was optional (and so it fails the definition of "tax" per the Internal Revenue Code).

Maria U. Ku, CPA

And then there are the PTE elective taxes, which are deductible to the PTE.

I'm not sure that this statement regarding employees not being covered by CASDI and opting in is correct. It is my understanding that all employees in CA must make mandatory contributions unless they are covered by a Voluntary Plan for Disability Insurance (VPDI). A VPDI can be offered by the employer in lieu of CASDI or the employees can vote for a VPDI. VPDI contributions are not tax deductible. The only individuals that I am aware of who can opt-in are the self-employed or business owners (not employees). That plan is called Disability Insurance Elective Coverage (DIEC). Those contributions are also not tax deductible.

Alan, Your statement "all employees in CA must make mandatory contributions unless they are covered by a Voluntary Plan for Disability Insurance (VPDI)" is incorrect. For example, as a community college faculty in California, I am not subject to SDI (neither to Social Security) tax. Maria U. Ku, CPA

Yes, I accidentally dropped the part that said "that were not exempt under the Insurance Code." See the following for those who are exempt.