NFC: Add a changelog entry for SE-0376.

Author: tshortli

CHANGELOG.md

@@ -4,6 +4,33 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 
 ## Swift 5.8
 
+* [SE-0376][]:
+
+  The `@backDeployed(before:)` attribute may now be used to extend the availability of a function to OS releases prior to the introduction of that function as ABI.
+  
+  For example, suppose that `struct Temperature` was introduced in a macOS SDK framework in macOS 12. Later in macOS 13 the framework authors decided to add a `degreesFahrenheit` property as a convenience:
+  
+  ```swift
+  @available(macOS 12, *)
+  public struct Temperature {
+    public var degreesCelsius: Double
+    
+    // ...
+  }
+  
+  extension Temperature {
+    @available(macOS 12, *)
+    @backDeployed(before: macOS 13)
+    public var degreesFahrenheit: Double {
+      return (degreesCelsius * 9 / 5) + 32
+    }
+  }
+  ```
+  
+  Adding the `@backDeployed` attribute to `degreesFahrenheit` enables the framework author to make this new declaration available to apps with a minimum deployment target of macOS 12, even though the ABI entry point for `degreesFahrenheit` is only present in macOS 13 and up.
+  
+  When a function with `@backDeployed` is called, the compiler wraps the invocation of the function in a thunk. The thunk checks whether the library entry point for the declaration is available at runtime, and invokes it if it is. Otherwise, a copy of the function that was emitted into the client is called instead.
+
 * [#56139][]:
 
   Сollection downcasts in cast patterns are now supported. For example:
@@ -9611,6 +9638,7 @@ using the `.dynamicType` member to retrieve the type of an expression should mig
 [SE-0362]: <https://github.com/apple/swift-evolution/blob/main/proposals/0362-piecemeal-future-features.md>
 [SE-0365]: <https://github.com/apple/swift-evolution/blob/main/proposals/0365-implicit-self-weak-capture.md>
 [SE-0370]: <https://github.com/apple/swift-evolution/blob/main/proposals/0370-pointer-family-initialization-improvements.md>
+[SE-0376]: <https://github.com/apple/swift-evolution/blob/main/proposals/0376-function-back-deployment.md>
 
 [#42697]: <https://github.com/apple/swift/issues/42697>
 [#42728]: <https://github.com/apple/swift/issues/42728>

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -53,17 +53,7 @@ Adding this attribute to a type leads to remarks being emitted for all methods.
 
 ## `@_backDeploy(before: ...)`
 
-Causes the body of a function to be emitted into the module interface to be
-available for emission into clients with deployment targets lower than the
-ABI availability of the function. When the client's deployment target is
-before the function's ABI availability, the compiler replaces calls to that
-function with a call to a thunk that checks at runtime whether the original
-library function is available. If the original is available then it is
-called. Otherwise, the fallback copy of the function that was emitted into the
-client is called instead.
-
-For more details, see the [pitch thread](https://forums.swift.org/t/pitch-function-back-deployment/55769/)
-in the forums.
+The spelling of `@backDeployed(before:)` prior to the acceptance of [SE-0376](https://github.com/apple/swift-evolution/blob/main/proposals/0376-function-back-deployment.md).
 
 ## `@_borrowed`