[swift-evolution] [Proposal] Remove force unwrapping in function signature.

[J.E. Schotsman]

The only reason I can think of writing a function with a force-unwrapped parameter is overriding an API function with such a signature, as Chris mentioned.

If the function actually doesn’t accept nil you might feel obliged to start the function with something like

precondition( IUO argument != nil, “this method cannot handle nil”)

in order to at least provide a message rather than just crashing.

As for consistency and symmetry I don’t think the current practice meets these criteria.
In your own code an IUO means “only use if you are sure it’s non-nil” whereas in imported APIs it means “enter nil at your own risk”.
If the unwrap shorthand `if let x!` would be accepted it would mean “if unwrap succeeds use as if it’s an IUO instead of an ordinary optional”. 
That would make three shades of weirdness. I like it.