[Pitch #3] Introduce user-defined dynamically "callable" types

[benrimmington]

@Chris_Lattner3

The answer is straightforward: we want to produce a compiler time error (not a runtime error) when someone uses (e.g.) keyword arguments with a language binding that does not support them (e.g. Javascript).

I've seen APIs which simulate argument labels, by taking an object literal.

// JavaScript

function example(args) {
  var first = args.first || 0 // default is zero.
  // etc.
}

example({first: 1, second: 2, third: 3})

So a JavaScript bridge might want to support both styles (arguments and keywordArguments).