@preconcurrency makes import order significant

[saagarjha]

Description

When using @preconcurrency the order of imports becomes significant. In particular, when importing a module with @preconcurrency, it sometimes must go first or its annotation will not take effect. I think this is because the annotation is somehow not being applied consistently to reexported modules?

Reproduction

import Foundation
@preconcurrency import Darwin

optind

Expected behavior

This should compile. Darwin is imported using @preconcurrency, which means that optind that Darwin exposes should be marked with it.

Environment

$ swiftc -version
swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.11 clang-1600.0.26.2)
Target: arm64-apple-macosx15.0

Additional information

For completeness, the following code compiles:

@preconcurrency import Darwin
import Foundation

optind

I had originally brought this up as a swift-format bug at swiftlang/swift-format#826 but @ahoppen thinks this is a compiler bug so I'm reporting it here instead.