Merge pull request #37254 from kavon/changelog-se310

Author: swift-ci

CHANGELOG.md

@@ -6,6 +6,50 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 Swift 5.5
 ---------
 
+* [SE-0310][]:
+  
+  Read-only computed properties and subscripts can now define their `get` accessor to be `async` and/or `throws`, by writing one or both of those keywords between the `get` and `{`.  Thus, these members can now make asynchronous calls or throw errors in the process of producing a value:
+  ```swift
+  class BankAccount: FinancialAccount {
+    var manager: AccountManager?
+
+    var lastTransaction: Transaction {
+      get async throws {
+        guard manager != nil else { throw BankError.notInYourFavor }
+        return await manager!.getLastTransaction()
+      }
+    }
+
+    subscript(_ day: Date) -> [Transaction] {
+      get async {
+        return await manager?.getTransactions(onDay: day) ?? []
+      }
+    }
+  }
+
+  protocol FinancialAccount {
+    associatedtype T
+    var lastTransaction: T { get async throws }
+    subscript(_ day: Date) -> [T] { get async }
+  }
+  ```
+  Accesses to such members, like `lastTransaction` above, will require appropriate marking with `await` and/or `try`:
+  ```swift
+  extension BankAccount {
+    func meetsTransactionLimit(_ limit: Amount) async -> Bool {
+      return try! await self.lastTransaction.amount < limit
+      //                    ^~~~~~~~~~~~~~~~ this access is async & throws
+    }                
+  }
+
+    
+  func hadWithdrawlOn(_ day: Date, from acct: BankAccount) async -> Bool {
+    return await !acct[day].allSatisfy { $0.amount >= Amount.zero }
+    //            ^~~~~~~~~ this access is async
+  }
+  ```
+
+
 * [SE-0306][]:
 
   Swift 5.5 includes support for actors, a new kind of type that isolates its instance data to protect it from concurrent access. Accesses to an actor's instance declarations from outside the must be asynchronous:
@@ -8433,6 +8477,7 @@ Swift 1.0
 [SE-0298]: <https://github.com/apple/swift-evolution/blob/main/proposals/0298-asyncsequence.md>
 [SE-0299]: <https://github.com/apple/swift-evolution/blob/main/proposals/0299-extend-generic-static-member-lookup.md>
 [SE-0306]: <https://github.com/apple/swift-evolution/blob/main/proposals/0306-actors.md>
+[SE-0310]: <https://github.com/apple/swift-evolution/blob/main/proposals/0310-effectful-readonly-properties.md>
 
 [SR-75]: <https://bugs.swift.org/browse/SR-75>
 [SR-106]: <https://bugs.swift.org/browse/SR-106>