The 35 Summilux ASPH (pre-FLE) is pretty flare-resistant. But it is not immune.
...and here's an example from the very first roll I shot with my ZM 21/4.5 Biogon-C:
After I put a multicoated filter in the 21/4.5 (to cover the chrome filter ring) it never did that again. However, I wanted a faster lens, so I replaced the 21/4.5 with a ZM 21/2.8. I have tried to get the 21/2.8 to flare, and have been unable to do so even without a filter covering the silver ring. It is probably the most flare-resistant lens I've ever used.