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skills/programming-swift/GuidedTour/Compatibility.md

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+# Version Compatibility
+
+Learn what functionality is available in older language modes.
+
+This book describes Swift 6.2.1,
+the default version of Swift that's included in Xcode 26.1.
+You can use the Swift 6.2.1 compiler to build code
+that's written in Swift 6.2.1, Swift 5, Swift 4.2, or Swift 4.
+
+When you use the Swift 6.2.1 compiler
+to build code that uses the Swift 5 language mode,
+you can use the new features from Swift 6.2.1 ---
+they're enabled either by default or by an upcoming feature flag.
+However, to enable strict concurrency checking,
+you need to upgrade to the Swift 6.2.1 language mode.
+
+In addition,
+when you use Xcode 15.3 to build Swift 4 and Swift 4.2 code,
+most Swift 5 functionality is still available.
+That said,
+the following changes are available only to code
+that uses the Swift 5 language mode:
+
+- Functions that return an opaque type require the Swift 5.1 runtime.
+- The `try?` expression doesn't introduce an extra level of optionality
+  to expressions that already return optionals.
+- Large integer literal initialization expressions are inferred
+  to be of the correct integer type.
+  For example, `UInt64(0xffff_ffff_ffff_ffff)` evaluates to the correct value
+  rather than overflowing.
+
+Concurrency requires the Swift 5 language mode
+and a version of the Swift standard library
+that provides the corresponding concurrency types.
+On Apple platforms, set a deployment target
+of at least iOS 13, macOS 10.15, tvOS 13, watchOS 6, or visionOS 1.
+
+A target written in Swift 6.2.1 can depend on
+a target that's written in Swift 5, Swift 4.2 or Swift 4,
+and vice versa.
+This means, if you have a large project
+that's divided into multiple frameworks,
+you can migrate your code to a newer language version
+one framework at a time.
+
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+
+Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+Licensed under Apache License v2.0 with Runtime Library Exception
+
+See https://swift.org/LICENSE.txt for license information
+See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+-->