Swift 6.2.3 (Xcode 26.2 CLI) cannot fresh-resolve a dependent with traits: ["Subprocess"] — "exhausted attempts to resolve the dependencies graph"

With the Xcode 26.2 default toolchain (Swift 6.2.3), a fresh swift package resolve (no Package.resolved) of any package depending on JSONFoundation with the Subprocess trait enabled fails deterministically:

error: exhausted attempts to resolve the dependencies graph, with the following dependencies unresolved:
* 'swift-subprocess' from https://github.com/swiftlang/swift-subprocess.git

Minimal repro (fails):

// swift-tools-version: 6.1
import PackageDescription
let package = Package(
    name: "probe",
    platforms: [.macOS("13.0")],
    dependencies: [
        .package(url: "https://github.com/Cocoanetics/JSONFoundation.git", from: "2.4.0", traits: ["Subprocess"])
    ],
    targets: [.target(name: "probe")]
)

Control experiments:

• A package depending on swift-subprocess from: "0.5.0" directly: resolves fine — the package itself is resolvable.
• The repro manifest plus a direct .package(url: swift-subprocess, from: "0.5.0") line: resolves fine — direct participation stabilizes the solver.

So the SwiftPM 6.2.3 solver fails specifically when swift-subprocess participates only via an enabled trait of a dependency — even though JSONFoundation declares the .package line unconditionally. Swift 6.3.1 (SwiftMCP CI) is unaffected; consumers with a valid lockfile are unaffected.

Impact: SwiftMCP itself (JSONFoundation with traits: ["Subprocess"]) cannot be fresh-resolved on the Xcode 26.2 toolchain — both swift package resolve and Xcode's own resolver fail. Bootstrap workaround: temporarily add the direct swift-subprocess dependency, resolve, remove it, keep the Package.resolved. Sibling of the resolver bug worked around in SwiftMCP d800f14. The real defect is SwiftPM's; this issue tracks it consumer-side (options: document the bootstrap, or declare swift-subprocess as a direct dependency until 6.2.x is irrelevant).

🤖 Generated with Claude Code

Start with the minimal Package.swift reproduction and run swift package resolve under Swift 6.2.3/Xcode 26.2, comparing it with the direct swift-subprocess dependency control. Check the resolver behavior for the enabled Subprocess trait and determine whether the issue should document the bootstrap workaround or track a direct dependency until the affected toolchain is no longer relevant.