Many income tax treaties are designed to mitigate "double taxation" by allowing a resident to take a credit for foreign tax paid on their domestic tax return. However, a resident would not be allowed to take a credit (on their domestic tax return) in excess of the "treaty rate" of tax, which is often 15% for individuals.
Thus, given that 35% Swiss tax was withheld from your Roche dividends, the use of a 15% foreign tax credit does not impact your ability to obtain a refund of the "excess" Swiss tax withheld calculated as: 35% Swiss tax withheld - 15% tax treaty rate for dividends = 20% "excess" tax withheld and recoverable from the Swiss Federal Tax Administration (FTA).