[Utility] Fix ordering expectation on versions.

 - This fixes a serious regression where `versions` was expected to be in
   order... it also boosts our test coverage of this scenario in two ways and
   adds an assertion on the internal invariant which wasn't previously
   documented.

Author: ddunbar

Sources/Get/RawClone.swift

@@ -103,7 +103,9 @@ class RawClone: Fetchable {
     }
 
     var availableVersions: [Version] {
-        return repo.versions
+        let versions = repo.versions
+        assert(versions == versions.sorted())
+        return versions
     }
 
     var finalName: String {

Sources/Utility/Git.swift

@@ -77,10 +77,10 @@ public class Git {
             return knownVersions
         }(self)
 
-        /// The set of versions in the repository.
-        public var versions: [Version] {
-            return [Version](knownVersions.keys)
-        }
+        /// The set of versions in the repository, in order.
+        public lazy var versions: [Version] = { repo in
+            return [Version](repo.knownVersions.keys).sorted()
+        }(self)
 
         /// Check if repo contains a version tag
         public var hasVersion: Bool {

Tests/FunctionalTests/DependencyResolutionTests.swift

@@ -38,8 +38,15 @@ class DependencyResolutionTestCase: XCTestCase {
         }
     }
 
+    /// Check resolution of a trivial package with one dependency.
     func testExternalSimple() {
-        fixture(name: "DependencyResolution/External/Simple") { prefix in
+        // This will tag 'Foo' with 1.0.0, to start.
+        fixture(name: "DependencyResolution/External/Simple", tags: ["1.0.0"]) { prefix in
+            // Add several other tags to check version selection.
+            for tag in ["1.1.0", "1.2.0", "1.2.3"] {
+                try tagGitRepo(prefix.appending(components: "Foo"), tag: tag)
+            }
+
             XCTAssertBuilds(prefix.appending(component: "Bar"))
             XCTAssertFileExists(prefix.appending(components: "Bar", ".build", "debug", "Bar"))
             XCTAssertDirectoryExists(prefix.appending(components: "Bar", "Packages", "Foo-1.2.3"))

Tests/UtilityTests/GitTests.swift

@@ -10,10 +10,14 @@
 
 import XCTest
 
-import TestSupport
 import Basic
+
+import struct PackageDescription.Version
+
 @testable import Utility
 
+import TestSupport
+
 class GitMoc: Git {
     static var mocVersion: String = "git version 2.5.4 (Apple Git-61)"
     override class var version: String! {
@@ -78,6 +82,16 @@ class GitUtilityTests: XCTestCase {
         }
     }
 
+    func testVersionOrdering() throws {
+        mktmpdir { dir in
+            let versionTags = (0..<10).map{ "\($0).0.0" }
+            initGitRepo(dir)
+            try versionTags.forEach{ try tagGitRepo(dir, tag: $0) }
+            let repo = Git.Repo(path: dir)!
+            XCTAssertEqual(repo.versions, versionTags.map{ Version($0)! })
+        }
+    }
+
 //MARK: - Helpers
 
     func checkSha(_ sha: String) {
@@ -100,5 +114,6 @@ class GitUtilityTests: XCTestCase {
         ("testVersionSha", testVersionSha),
         ("testHeadAndVersionSha", testHeadAndVersionSha),
         ("testHasLocalChanges", testHasLocalChanges),
+        ("testVersionOrdering", testVersionOrdering),
     ]
 }