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kotlinlang

I know I could just omit the parameter but then I would have to do:
```
if (s == null) foo() else foo(s)
```

Not sure default value helps you here, within `foo` you could
```
fun foo(input: String?)  {
    val s = input ?: "default"
}
```

Or maybe do both
```
fun foo(input: String? = "default")  {
    val s = input ?: "default"
}
```

```
fun foo(input: String? = "hello"): String {
            return input ?: "default"
        }

        assert(foo() == "hello")
        assert(foo(null) == "default")
        assert(foo("bar") == "bar")
```
This illustrates the behaviour you can expect

if you provide the same value as parameter default and null elvis fallback, you get the desired behaviour, as described in OP

that is,
```fun foo(input: String? = "default"): String {
    return input ?: "default"
}

assert(foo() == "default")
assert(foo(null) == "default")
assert(foo("bar") == "bar")
```