Add test case that implicit self in a weak closure doesn't result in a strong capture

Author: calda

test/expr/implicit_weak_capture.swift

@@ -0,0 +1,29 @@
+// RUN: %target-run-simple-swift(-Xfrontend -disable-availability-checking)
+
+// REQUIRES: executable_test
+
+func runIn10ms(_ closure: @escaping @Sendable () -> Void) {
+  Task {
+    try! await Task.sleep(nanoseconds: 10_000_000)
+    closure()
+  }
+}
+
+final class Weak: Sendable {
+  let property = "Self exists"
+
+  func test() {
+    runIn10ms { [weak self] in
+      if let self {
+        // Use implicit self -- this should not result in a strong capture
+        _ = property
+        fatalError("Self was unexpectedly captured strongly")
+      } else {
+        print("Self was captured weakly")
+      }
+    }
+  }
+}
+
+Weak().test()
+try await Task.sleep(nanoseconds: 20_000_000)