[stdlib] Roll back generalization of the nil-coalescing operator ??

We cannot currently express its proper lifetime semantics: its result’s lifetime should depend on the intersection of the lifetime of the left argument and the lifetime of the result of the right argument.

`@lifetime(optional, defaultValue.result)` is what we want, but the `.result` part is not currently expressible. (Tying the dependency on the closure argument itself may be a viable option, but we aren’t confident enough to ship it like that yet.)

Author: lorentey

stdlib/public/core/Optional.swift

@@ -809,11 +809,15 @@ extension Optional where Wrapped: ~Copyable & ~Escapable {
 ///     type as the `Wrapped` type of `optional`.
 @_transparent
 @_alwaysEmitIntoClient
-@lifetime(optional)
-public func ?? <T: ~Copyable & ~Escapable>(
+public func ?? <T: ~Copyable>(
   optional: consuming T?,
-  defaultValue: @autoclosure () throws -> T // FIXME: typed throw
+  defaultValue: @autoclosure () throws -> T // FIXME: typed throws
 ) rethrows -> T {
+  // FIXME: We want this to support nonescapable `T` types.
+  // To implement that, we need to be able to express that the result's lifetime
+  // is limited to the intersection of `optional` and the result of
+  // `defaultValue`:
+  //    @lifetime(optional, defaultValue.result)
   switch consume optional {
   case .some(let value):
     return value
@@ -881,12 +885,15 @@ internal func _legacy_abi_optionalNilCoalescingOperator <T>(
 ///     `optional` have the same type.
 @_transparent
 @_alwaysEmitIntoClient
-@lifetime(optional)
-// FIXME: This needs to support typed throws.
-public func ?? <T: ~Copyable & ~Escapable>(
+public func ?? <T: ~Copyable>(
   optional: consuming T?,
-  defaultValue: @autoclosure () throws -> T?
+  defaultValue: @autoclosure () throws -> T? // FIXME: typed throws
 ) rethrows -> T? {
+  // FIXME: We want this to support nonescapable `T` types.
+  // To implement that, we need to be able to express that the result's lifetime
+  // is limited to the intersection of `optional` and the result of
+  // `defaultValue`:
+  //    @lifetime(optional, defaultValue.result)
   switch consume optional {
   case .some(let value):
     return value

test/api-digester/Outputs/stability-stdlib-source-base.swift.expected

@@ -233,7 +233,7 @@ Constructor UnsafeRawPointer.init(_:) has generic signature change from <T> to <
 Enum MemoryLayout has generic signature change from <T> to <T where T : ~Copyable, T : ~Escapable>
 Enum Optional has generic signature change from <Wrapped> to <Wrapped where Wrapped : ~Copyable, Wrapped : ~Escapable>
 Enum Result has generic signature change from <Success, Failure where Failure : Swift.Error> to <Success, Failure where Failure : Swift.Error, Success : ~Copyable, Success : ~Escapable>
-Func ??(_:_:) has generic signature change from <T> to <T where T : ~Copyable, T : ~Escapable>
+Func ??(_:_:) has generic signature change from <T> to <T where T : ~Copyable>
 Func ??(_:_:) has parameter 0 changing from Default to Owned
 Func ManagedBuffer.create(minimumCapacity:makingHeaderWith:) has generic signature change from <Header, Element> to <Header, Element where Element : ~Copyable>
 Func ManagedBufferPointer.isUniqueReference() has generic signature change from <Header, Element> to <Header, Element where Element : ~Copyable>