[SR-11017] dynamicallyCall can't be mutating

[mattneub]

Previous ID	SR-11017
Radar	None
Original Reporter	@mattneub
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement
Assignee	None
Priority	Medium

md5: 0d961a467f1fb47f1346ba53df523c0f

Issue Description:

@dynamicCallable struct S {
    func dynamicallyCall(withKeywordArguments kvs:KeyValuePairs<String, String>) {}
}
var s = S()
s(testing:"testing")

That compiles. But now suppose our dynamically called method is supposed to mutate the struct. If we put `mutating` in front of `func`, we get a compile error: "cannot call value of non-function type 'S'".

I can work around this by making this a class instead of a struct. For example, this works:

@dynamicCallable class Flock {
    var d = ["partridge":"in a pear tree"]()
    func dynamicallyCall(withArguments: [String]) {}
    func dynamicallyCall(withKeywordArguments kvs:KeyValuePairs<String, String>) {
        if kvs.count == 1 {
            let (key,val) = kvs.first!
            switch key {
            case "remove": d[val] = nil
            default: break
            }
        }
    }
}
var flock = Flock()
flock(remove:"partridge")

But if Flock is a struct, I can't find any way to do that.

[theblixguy]

cc @dan-zheng

[dan-zheng]

Supporting for mutating func dynamicallyCall should be possible.
Similar support for mutating func callAsFunction is being implemented in #24299
There are still some bugs (namely with mutating func callAsFunction + IUO), but once done, the approach can be ported to dynamicallyCall.
I'm not sure whether this change to dynamicallyCall warrants any Swift Evolution discussion - I imagine it's noncontroversial.

[mattneub]

Surely a struct method that cannot be marked `mutating` and called on a `var` reference to that struct is not Swift.