@devincoughlin, @rjmccall, any insights?

The type-specificity suggests that this is related to the code-generation differences for an indirect-out parameter, but I don't know why. Regardless, clearly a bug.

Ah, it's the reabstraction thunk.

Reduced test case:

extension Array {
    mutating func update(_ f: () -> (Element)) {}
}

func test(xs: inout [Int], ys: inout [Int]) {
    xs.update {
        ys = xs
        return 3
    }
}

Probably possible to address this directly with the current representation, but might be easier to fix if we had a reabstract_function instruction.

Tracking internally as rdar://35215926.

There is a fix in #12929

Should be fixed in #12929

Thanks for reporting the issue!

Thanks to fix it.
I am grad to report it.

Sorry, why am I set to assignee ?
What should I do?

We assign the bug to the originator so you can confirm that the the bug is fixed and close the JIRA issue. If you have time, it would be helpful if you could download a nightly toolchain from swift.org and confirm that the bug is fixed. This helps us make sure that we fixed the bug properly. If you don't have the time, then you can close the JIRA issue.

Thanks Devin to explain for me.
I understand operation.

I checked with snapshot at 2017/11/21.

Unfortunately compiler is still wrong a little.
It reports conflict correctly, but it say that is warning.

Of course If I change `Element` to `Int` at line 2,
Compiler say that is error.

This is log.

[omochi@omochi-iMac bbb]$ cat a.swift
extension Array {
    mutating func update(_ f: () -> Element) {}
}

func main() {
    var xs: [Int] = [1, 2, 3]
    var ys: [Int] = []
    xs.update {
        ys = xs
        return 3
    }
}

main()
[omochi@omochi-iMac bbb]$ export TOOLCHAINS=org.swift.3020171121a
[omochi@omochi-iMac bbb]$ swift --version
Apple Swift version 4.1-dev (LLVM fe49d8f2ca, Clang 839070845c, Swift c0c04864db)
Target: x86_64-apple-darwin16.7.0
[omochi@omochi-iMac bbb]$ swift a.swift
a.swift:8:9: warning: variable 'ys' was written to, but never read
    var ys: [Int] = []
        ^
a.swift:9:5: warning: overlapping accesses to 'xs', but modification requires exclusive access; consider copying to a local variable
    xs.update {
    ^~
a.swift:10:12: note: conflicting access is here
        ys = xs
        ~~~^~~~

Thanks for for checking this.

For Swift 4.1 the new diagnostics will be warnings and not errors. This is for source compatibility. Where possible, we would like to avoid code that compiles with the 4.0 compiler having an error in 4.1. The plan is to turn the new diagnostics into hard errors in a future version of the Swift compiler.

Oh I see. I close issue.

thank you