swift-module-compat-test

A set of bare-bones repro projects for https://bugs.swift.org/browse/SR-11704

Background

In November 2019, a user reported https://bugs.swift.org/browse/SR-11704, which describes the following scenario:

• Build a binary framework in version X of Swift, where X >= 5.1
• Consume the binary framework in a project that uses version Y of Swift, where Y > X

Expected results: Because Swift introduced module compatibility with Swift 5.1, it is expected that projects would be able to consume binary frameworks built with earlier versions of Swift.

Actual results: Attempting to build the consuming project with the binary framework built using an earlier version of Swift results in a compiler error: 'QuoteList' is not a member type of 'QuoteProvider'

Project structure

This GitHub repo contains two projects:

• QuoteProvider: A module that has the BUILD_LIBRARY_FOR_DISTRIBUTION flag enabled, which turns on Module Compatibility. (See the "frozen" section of https://docs.swift.org/swift-book/ReferenceManual/Attributes.html). For convenience, this project also contains a Carthage directory that hosts binary frameworks built with Xcode 11.7/Swift 5.2.4.
• QuoteApp: An app that consumes the QuoteProvider framework as a binary artifact. For convenience, this project includes a Framework directory that hosts the QuoteProvider.framework binary dependency

Repro steps

Using the pre-installed binaries

• On an Intel Mac, open QuoteApp.xcodeproj in Xcode 12 or higher (Swift 5.3). This project already contains a copy of the binary QuoteProvider.framework from the QuoteProvider project, built with Xcode 11.4 (Swift 5.2.4)
• Build the project

From scratch

• Select a version of Xcode that includes a Swift toolchain of version 5.1 or greater, but less than whatever version you're going to build your app with:

  sudo xcode-select --switch /Applications/Xcode_11.7.app

• Build the QuoteProvider project. For convenience, the QuoteProvider project includes a build script that invokes Carthage to do this:

  ./build-support/carthage-build.sh --no-skip-current

  Alternately, you can invoke xcodebuild directly with appropriate options.

• Move the generated QuoteProvider.framework into the QuoteApp's "Frameworks" directory, replacing the existing QuoteProvider.framework

• Select a version of Xcode that includes a Swift toolchain of greater than whatever version you built the framework with:

  sudo xcode-select --switch /Applications/Xcode_12_2.app

• Open QuoteApp.xcodeproj in the newer version of Xcode

• Build the project

Discussion

This is a simple repro case for the bug above. The QuoteProvider project declares a Swift module named "QuoteProvider". It contains two public types: a QuoteProvider and a QuoteList. One of the public methods on QuoteProvider returns a QuoteList. The module interface for this looks like:

// swift-interface-format-version: 1.0
// swift-compiler-version: Apple Swift version 5.2.4 (swiftlang-1103.0.32.9 clang-1103.0.32.53)
// swift-module-flags: -target x86_64-apple-ios9.0-simulator -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -module-name QuoteProvider
import Foundation
@_exported import QuoteProvider
import Swift
@_hasMissingDesignatedInitializers public class QuoteProvider {
  public static func getQuote() -> Swift.String
  public static func getQuoteList() -> QuoteProvider.QuoteList
  @objc deinit
}
@_hasMissingDesignatedInitializers public class QuoteList {
  public func getQuoteList() -> [Swift.String]
  @objc deinit
}

The bug seems to result because Swift interprets the module QuoteProvider as the name of the type QuoteProvider, so the return value of getQuoteList is ambiguous: During compilation, Swift expects the type QuoteProvider to contain a sub-type QuoteList. But the module definition is actually declaring that the method is returning a QuoteList type from the module named "QuoteProvider".