Handling C++ exceptions

[Chris_Lattner3 (Chris Lattner)]

@gribozavr

Right -- what you are describing is indeed our tentative plan. The question is, what is the best implementation strategy to ensure program termination when it attempts to unwind through a Swift frame -- a strategy that would impose minimal performance and code size overhead on Swift code calling C++. Martin proposed a custom personality function, but maybe there's an easier way.

From a type system perspective, C++ (and python) functions would be best modeled with something like a throws! marker. Something that can be ignored in a non-try context but that trap when ignored, or that can be used with try if/when a swift programmer explicitly want to be able to handle the C++ exception, explicitly turning the exception into a Swift error.

-Chris