Allow parameters to default to the value of earlier parameters

[Jumhyn (Frederick Kellison-Linn)]

I recently found myself wanting this as well, and posted this thread. Wasn’t able to find a previous discussion but glad this got surfaced again!

Allow default argument expressions to reference earlier parameters

Occasionally, I find myself wanting to write something such as: func f(x: Int, y: Int = x) { // 'y' defaults to whatever is passed for 'x' print(x, y) } AFAIU, Swift guarantees argument evaluation order from left to right (though I couldn't find a reference for this... so I could be misremembering?), so I believe that letting default argument expressions refer to previous parameters should be well defined. A call such as f(x: 0) would implicitly expand to something like: let _x = 0 f(x: _x,…

The discussion there goes into the some of the problems this feature would have to contend with w.r.t. the current model for default arguments.