[SR-10007] Casting error with tuples

[swift-ci]

Previous ID	SR-10007
Radar	None
Original Reporter	Tainnor (JIRA User)
Type	Bug

Environment

Apple Swift version 4.2.1 (swiftlang-1000.11.42 clang-1000.11.45.1)
Target: x86_64-apple-darwin18.2.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, RunTimeCrash, Runtime, SILGen
Assignee	None
Priority	Medium

md5: b136ffa2ceaa3e278adf2b4bdf1b810d

Issue Description:

The following code compiles fine but throws an unhelpful runtime error:

private func liftToEnumerated<A1, A2, B>(_ f: @escaping (A1, A2) -> B) -> ((Int, A1), (Int, A2)) -> B {
    return { enumerated1, enumerated2 in f(enumerated1.1, enumerated2.1) }
}

private func liftAllToEnumerated<A1, A2, B>(_ fs: [(A1, A2) -> B]) -> [((Int, A1), (Int, A2)) -> B] {
    return fs.map(liftToEnumerated)
}

extension Sequence {
    func sorted(byMultiple predicates: [(Element, Element) -> Bool]) -> [Self.Element] {
        let combinedPredicate = { (e1: Element, e2: Element) -> Bool in
            return predicates.reduce(false) { areInIncreasingOrder, predicate in
                return areInIncreasingOrder || predicate(e1, e2)
            }
        }

        return sorted(by: combinedPredicate)
    }
}

let ps: [(Int, Int) -> Bool] = [{_, _ in true}]
// this is fine:
[1,2,3].sorted(byMultiple: ps)
// this crashes:
[1,2,3].enumerated().sorted(byMultiple: liftAllToEnumerated(ps))

The error message is (in the REPL, but it also crashes outside of the REPL):

Could not cast value of type '((Swift.Int, Swift.Int), (Swift.Int, Swift.Int)) -> Swift.Bool' (0x1013d29b8) to '((offset: Swift.Int, element: Swift.Int), (offset: Swift.Int, element: Swift.Int)) -> Swift.Bool' (0x1013d2cc0).
2019-02-27 16:42:18.871464+0100 repl_swift[65408:923280] Could not cast value of type '((Swift.Int, Swift.Int), (Swift.Int, Swift.Int)) -> Swift.Bool' (0x1013d29b8) to '((offset: Swift.Int, element: Swift.Int), (offset: Swift.Int, element: Swift.Int)) -> Swift.Bool' (0x1013d2cc0).
Execution interrupted. Enter code to recover and continue.
Enter LLDB commands to investigate (type :help for assistance.)
Process 65408 stopped
* thread #​1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
    frame #​0: 0x00007fff7879623e libsystem_kernel.dylib`__pthread_kill + 10
libsystem_kernel.dylib`__pthread_kill:
->  0x7fff7879623e <+10>: jae    0x7fff78796248            ; <+20>
    0x7fff78796240 <+12>: movq   %rax, %rdi
    0x7fff78796243 <+15>: jmp    0x7fff787903b7            ; cerror_nocancel
    0x7fff78796248 <+20>: retq
Target 0: (repl_swift) stopped.

I don't really understand what's going on here.

[belkadan]

The compiler's inserting a cast that removes tuple labels from within a function type, but the runtime doesn't know how to do that. I think your code is valid but it's certainly relying on that; you could match the return type of enumerated() to not rely on that:

private func liftToEnumerated<A1, A2, B>(_ f: @escaping (A1, A2) -> B) -> ((offset: Int, element: A1), (offset: Int, element: A2)) -> B {
    return { enumerated1, enumerated2 in f(enumerated1.1, enumerated2.1) }
}

private func liftAllToEnumerated<A1, A2, B>(_ fs: [(A1, A2) -> B]) -> [((offset: Int, element: A1), (offset: Int, element: A2)) -> B] {
    return fs.map(liftToEnumerated)
}

cc @xedin, @slavapestov