Swift 4 minimal macOS compatibility

Depends what you're asking. You might be able to run Swift 4 itself (that is, the toolchain) on older macOS versions for non-Xcode development.

If you're asking what's the oldest deployment target, the oldest macOS that Swift-compiled apps will run on, the answer is here:

https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/index.html

That is, Swift can deploy back to macOS 10.9.

If you're asking what's the oldest macOS to run Swift 4 within Xcode, you need Xcode 9, which needs to run on 10.12.4+, which forces you to SDK 10.13+.

Swift 4 is Xcode 9.x - minimum macOS for Xcode 9, as noted, is 10.12.4 - I would not expect earlier macOSs to 'run' Swift 4, at least not by default.

The Xcode wiki has other examples:

h ttps://en.wikipedia.org/wiki/Xcode

From swift.org:

https://swift.org/download/#releases

Requirements for Tools

Swift 4.0

• macOS 10.12.4 (El Capitan)
• Xcode 9.0 beta or later

Swift 3.1

• macOS 10.11.5 (El Capitan)
• Xcode 8.3 - 8.3.3

Swift 3.0

• macOS 10.11.5 (El Capitan)
• Xcode 8.0 - 8.2

Swift 2.2.x

• macOS 10.11 (El Capitan)
• Xcode 7.2 - 7.3

This section of "The Swift Programming Language" has been inexplicably removed from the Swift 4.0.3 version of the manual, and I cannot locate any current definitive statement of minimum target version compatibility, which seems unnecessarily confusing.

For future reference, the missing section read:

• Creating an app using Swift 4.0 requires Xcode 9.0 or newer, as well as the following base SDK requirements: [macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0]
• The Swift compiler and Xcode enforce a minimum deployment target of iOS 7 or macOS 10.9. Setting an earlier deployment target results in a build failure.

This section of "The Swift Programming Language" has been inexplicably removed from the Swift 4.0.3 version of the manual, and I cannot locate any current definitive statement of minimum target version compatibility, which seems unnecessarily confusing.

Quite. Please file a bug against the docs requesting that this info be reinstated.

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—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

I finally got around to filing this (rdar://41373126).

The Xcode wiki may have the details you're looking for.