Two dependencies need incompatible versions of a third. What does the resolver say, and how do you get out?

Predict first

What does the resolver report when A needs Lib 1.x and B needs Lib 2.x?

// === run.sh ===
export GIT_AUTHOR_NAME=t GIT_AUTHOR_EMAIL=t@t GIT_COMMITTER_NAME=t GIT_COMMITTER_EMAIL=t@t
W=$(mktemp -d); cd $W
mk() { mkdir -p $1/Sources/$1; (cd $1; git init -q -b main
  cat > Package.swift <<E
// swift-tools-version: 6.0
import PackageDescription
let package = Package(name: "$1", products: [.library(name: "$1", targets: ["$1"])], dependencies: [$3],
  targets: [.target(name: "$1", dependencies: [$4])])
E
  echo "public let $1Version = \"$2\"" > Sources/$1/V.swift; git add -A; git commit -qm $2; git tag $2); }
mk Lib 1.0.0 "" ""
(cd Lib; echo 'public let more = 1' > Sources/Lib/M.swift; git add -A; git commit -qm 2; git tag 2.0.0)
mk A 1.0.0 ".package(url: \"file://$W/Lib\", from: \"1.0.0\")" ".product(name: \"Lib\", package: \"Lib\")"
mk B 1.0.0 ".package(url: \"file://$W/Lib\", from: \"2.0.0\")" ".product(name: \"Lib\", package: \"Lib\")"
mkdir -p App/Sources/App; cd App
cat > Package.swift <<E
// swift-tools-version: 6.0
import PackageDescription
let package = Package(name: "App",
  dependencies: [.package(url: "file://$W/A", from: "1.0.0"), .package(url: "file://$W/B", from: "1.0.0")],
  targets: [.executableTarget(name: "App", dependencies: ["A", "B"])])
E
echo 'import A' > Sources/App/main.swift
swift package resolve 2>&1 | grep -E "^error|incompatible|practically" | sed "s#$W#.#g"

• An error naming b as incompatible with a, with lib 1.0.0..<2.0.0 against 2.0.0..<3.0.0
• It silently picks 2.0.0 and A breaks later
• It builds both versions of Lib side by side