Implicit strong captures of weak capture list items (ImplicitStrongCapture)

Overview

When you use a weak or unowned capture list item in a closure, but that object is implicitly captured as a strong reference in an outer escaping closure, the compiler emits a warning. This warning highlights cases that can cause unexpected reference cycles or unintended object lifetime extension.

For example, consider a hypothetical DataService class that uses a callback-based API to fetch data from the network, store the response model into a cache, and finally notify clients of an update:

final class DataService: Sendable {
  func fetchData(using cache: Cache) {
    networkService.fetch { responseData in
      let result = DataModel(responseData)
      cache.store(result) { [weak self] in
        self?.notifyClients(with: result)
      }
    }
  }
}

Here, self might appear to be weakly retained during the call to fetch, since it's only captured weakly in the inner closure. However, the presence of the capture list item causes the outer closure to capture a strong reference to self. To highlight this, the compiler emits a warning noting the ownership mismatch:

| final class DataService: Sendable {
|   func fetchData(using cache: Cache) {
|     networkService.fetch { responseData in
|                          |- note: 'self' implicitly strongly captured here
|                          `- note: add 'self' as a capture list item to silence
|       let result = DataModel(responseData)
|       cache.store(result) { [weak self] in
|                                   |- warning: 'weak' ownership of capture 'self' differs from implicitly-captured strong reference in outer scope
|                                   `- note: explicitly assign the capture list item to silence
|         self?.notifyClients(with: result)
|       }

To resolve the warning, add an explicit capture list item to the outer closure specifying the intended ownership. You can also silence the warning by explicitly assigning a value to the inner closure's capture list item (e.g. [weak x = x]).