Have the compiler generate thunks to populate default function parameters

[Erica_Sadun]

To eliminate the exponential explosion, one could burn a new keyword. I dunno, call it @applicable.

• You could not use this approach for any function declared with more than one undefaulted argument.
• For any f(...) -> T, where the number of undefaulted arguments is one, would use use the undefaulted argument as $0. For example: func print(_ items: Any..., separator: String = default, terminator: String = default), $0 would correspond to items, allowing you to pass print instead of { print($0) }
• For any f(...) -> T with all defaulted arguments, you could decorate precisely one argument with the keyword, for example: g(a: U = _x_, b: @applicable V = _y_) -> T, $0 would correspond to b, allowing you to pass g instead of { g(b: $0) }