~Copyable and @dynamicmemberlookup

Development Compiler
1

Playing around with wrapping some of my value types in noncopyable types generated the following in a playground under Xcode 15.1b2:

struct A { }
let a = A()

@dynamicMemberLookup
struct Ref<S>: ~Copyable {
    var value: S
    subscript<T>(dynamicMember keyPath: KeyPath<S, T>) -> T {
        value[keyPath: keyPath]
    }
}
let refA = Ref(value: a)

This produces:

error: couldn't IRGen expression. Please enable the expression log by running "log enable lldb expr", then run the failing expression again, and file a bug report with the log output.

Not quite sure a) how to do that last step or b) if this is supposed to work at this point. Thoughts?

2

ok, weird behavior from a playground. Here's a more precise version from trying to compile as a command line tool on Mac:

struct A { var a: String }

@dynamicMemberLookup
struct Ref<S>: ~Copyable {
    var value: S
    init(value: S) {
        self.value = value
    }
    subscript<T>(dynamicMember keyPath: KeyPath<S, T>) -> T {
        value[keyPath: keyPath]
    }
}

let a = A(a: "Yo")
let refA = Ref(value: a)
_ = refA.value.a
_ = refA.value[keyPath: \.a]
_ = refA.a

print("hello, world!")

The line using the dynamicMember produces:

Copy of noncopyable typed value. This is a compiler bug. Please file a bug with a small example of the bug

Is this supposed to work yet?

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3

Whether or not it's "supposed to work yet," it's worth a bug report. Any internal compiler diagnostic merits a bug report.

In this case, it appears to work on the nightly, so I think this has been fixed since 5.9 was cut? Compiler Explorer

CC @Joe_Groff @kavon

2 Likes