It looks like each call to recursive is getting its own independent mutable dictionary. Oddly enough, forcing cache to be a computed property fixes the problem as well:

func fibonacci(_ n: Int) -> Int {
    var cache: [Int: Int] = [:] {
      willSet { print(newValue) }
    }
    
    func recursive(_ m: Int) -> Int {
        return cache[m] ?? {
            let output = m < 2 ? m : recursive(m - 1) + recursive(m - 2)
            cache[m] = output
            print("cached \(m)")
            return output
        }()
    }
    return recursive(n)
}
_ = fibonacci(5)

@slavapestov, @jckarter, any idea how the captures are getting messed up?

Comment by Tim (JIRA)

@belkadan I think you posted the wrong snippet as it does not contain a computed property (but I can confirm that having a computed property `cache` that forwards to a property `_cache` fixes it).

Hm, for some reason we're creating an entirely new box for `cache` in the autoclosure for the right-hand side of ??:

// implicit closure #​1 in recursive #​1 (_:) in fibonacci(_:)
sil private [transparent] @$S3bar9fibonacciyS2iF9recursiveL_yS2iFSiyKXKfu_ : $@convention(thin) (Int, @guaranteed { var Dictionary<Int, Int> }, @inout_aliasable Dictionary<Int, Int>) -> (Int, @error Error) {
// %0                                             // users: %13, %4
// %1                                             // user: %5
// %2                                             // users: %13, %10, %7
bb0(%0 : $Int, %1 : ${ var Dictionary<Int, Int> }, %2 : $*Dictionary<Int, Int>):
  debug_value undef : $Error, var, name "$error", argno 1 // id: %3
  debug_value %0 : $Int, let, name "m", argno 2   // id: %4
  %5 = project_box %1 : ${ var Dictionary<Int, Int> }, 0 // user: %6
  debug_value_addr %5 : $*Dictionary<Int, Int>, var, name "cache", argno 3 // id: %6
  debug_value_addr %2 : $*Dictionary<Int, Int>, var, name "cache", argno 4 // id: %7
  %8 = alloc_box ${ var Dictionary<Int, Int> }    // users: %15, %14, %11, %9
  %9 = project_box %8 : ${ var Dictionary<Int, Int> }, 0 // user: %10
  copy_addr %2 to [initialization] %9 : $*Dictionary<Int, Int> // id: %10

@swift-ci create

Ah, I suppose it's not a property, but cache will be treated as computed because of the willSet.

Comment by Tim (JIRA)

@belkadan For what it's worth, your code snippet doesn't fix the problem in the 7/30 master snapshot (but it does in whatever Swift version Xcode 10 beta 5 ships with).

Huh, interesting. Well, either way we need to track down the real problem.

Computing the transitive capture list of the closure it needs `cache` for itself and for its calls to `recursive()` but with different capture flags so it ends up in the lowered capture info twice. Making `cache` a computed property keeps it from being direct, so the capture flags match, avoiding the problem.

#18514

The fix is merged into master, so newer snapshots ought to have this fix. I'm going to try to bring it into the 4.2 branch as well: #18701