[swift-evolution] [Idea] Use optionals for non-optional parameters

[Justin Jia]

Hi!

I don’t know if this has came up before. I tried to search though the mailing list but didn’t find any related threads.

This is purely a syntactic thing (which I know it’s the lowest priority for Swift 4), but I think it’s an important one.

Let’s say we have a struct with a function:

```
struct Foo {
    func bar(x: Int)
}
```

We can use optionals:

```
let foo: Foo? = nil
let x = 1
foo!.bar(x: x) // Able to compile, but will cause runtime error
foo?.bar(x: x) // Able to compile, and won't cause runtime error
```

However:

```
let foo = Foo()
let x: Int? = nil
foo.bar(x: x!) // Able to compile, but will cause runtime error
foo.bar(x: x?) // Won't compile
```

I propose that we should allow `foo.bar(x: x?)`, which should be equivalent to:

```
if let x = x {
  foo.bar(x: x)
}
```

What do you think?

Thanks,
Justin