Package-level module aliases are not applied

[fumoboy007]

Description

I have a package that needs to alias a dependency’s module due to conflicting module names.

Currently, I am doing this at the product level like so:

.product(name: "MessagePack", package: "MessagePack", moduleAliases: [
   "MessagePack": "FlightSchoolMessagePack",
]),

This works, but I would like to do the alias at the package level like so:

let flightSchoolMessagePackDependency: PackageDescription.Package.Dependency =
   .package(url: "https://github.com/Flight-School/MessagePack.git", from: "1.2.4")
flightSchoolMessagePackDependency.moduleAliases = ["MessagePack": "FlightSchoolMessagePack"]

let package = Package(
   …
   dependencies: [
      flightSchoolMessagePackDependency,
   …
)

Expected behavior

I expect the package-level module alias to be applied, similar to the product-level module alias.

Actual behavior

The package manager returns an error:

multiple targets named 'MessagePack' in: 'messagepack', 'msgpack-swift'; consider using the moduleAliases parameter in manifest to provide unique names

Steps to reproduce

1. Download my package.
2. Switch to the with-flight-school-dep branch.
3. Replace the dependency .package(url: "https://github.com/Flight-School/MessagePack.git", from: "1.2.4") with a similar dependency that has the moduleAliases property specified.
4. Remove the moduleAliases parameter from .product(name: "MessagePack", package: "MessagePack", moduleAliases: …).
5. swift build

Swift Package Manager version/commit hash

swift-5.8.1-RELEASE

Swift & OS version (output of swift --version ; uname -a)

swift-driver version: 1.75.2 Apple Swift version 5.8.1 (swiftlang-5.8.0.124.5 clang-1403.0.22.11.100)
Target: arm64-apple-macosx13.0
Darwin Darrens-Mac.momahome 22.6.0 Darwin Kernel Version 22.6.0: Wed Jul  5 22:22:05 PDT 2023; root:xnu-8796.141.3~6/RELEASE_ARM64_T6000 arm64

[fumoboy007]

@elsh @tomerd Perhaps related to #6913? This one is 100% reproducible though.

[neonichu]

Do I understand correctly that this compiles?

let flightSchoolMessagePackDependency: PackageDescription.Package.Dependency =
   .package(url: "https://github.com/Flight-School/MessagePack.git", from: "1.2.4")
flightSchoolMessagePackDependency.moduleAliases = ["MessagePack": "FlightSchoolMessagePack"]

I would not expect that, IIRC we only defined module aliases at the product dependency level? cc @tomerd

[fumoboy007]

@neonichu The package-level property is defined here.

[elsh]

It was meant to be used as a property of a product dependency. It was declared public in Package.Dependency at the time it was added due to the need for intra-module visibility; there are other properties such as .targetItem(name:...) that were made public for the same reason even though they weren't meant to be visible to users. However, as of swift 5.9, a new access level called package is introduced, which limits visibility of such properties, so they can be hidden from users once modified with that access level.

[fumoboy007]

Hmm I guess we could/should have prefixed those symbols with an underscore to make that clear.

When are we changing the access level to package? Shall we repurpose this issue to track that work?

[neonichu]

I don't think we can do that until the next release after 5.10, but we should be able to use @spi here in the meantime, I think?

[neonichu]

Actually seems like we can just make it internal. It may have been needed to be public because of how the old serialization code worked that I refactored in 5.9