Fix use of XCTest minimum deployment target (#6504)

If the computed XCTest minimum deployment target is lower than the oldest supported version of a platform, we should not use it as the basis for deployment targets of test targets.

This issue was actually also present in the tests which were written to expect the incorrect behavior of always preferring the XCTest minimum deployment target.

rdar://108462045

Author: neonichu

Sources/PackageGraph/PackageGraph+Loading.swift

@@ -737,7 +737,7 @@ private func computePlatforms(
         let platform = platformRegistry.platformByName[platformName]!
 
         let minimumSupportedVersion: PlatformVersion
-        if usingXCTest, let xcTestMinimumDeploymentTarget = xcTestMinimumDeploymentTargets[platform] {
+        if usingXCTest, let xcTestMinimumDeploymentTarget = xcTestMinimumDeploymentTargets[platform], xcTestMinimumDeploymentTarget > platform.oldestSupportedVersion {
             minimumSupportedVersion = xcTestMinimumDeploymentTarget
         } else {
             minimumSupportedVersion = platform.oldestSupportedVersion

Tests/PackageGraphTests/PackageGraphTests.swift

@@ -2162,10 +2162,18 @@ class PackageGraphTests: XCTestCase {
         let customXCTestMinimumDeploymentTargets = [
             PackageModel.Platform.macOS: PlatformVersion("10.15"),
             PackageModel.Platform.iOS: PlatformVersion("11.0"),
-            PackageModel.Platform.tvOS: PlatformVersion("11.0"),
+            PackageModel.Platform.tvOS: PlatformVersion("10.0"),
             PackageModel.Platform.watchOS: PlatformVersion("4.0"),
         ]
 
+        let expectedPlatformsForTests = customXCTestMinimumDeploymentTargets.reduce(into: [Platform : PlatformVersion]()) { partialResult, entry in
+            if entry.value > entry.key.oldestSupportedVersion {
+                partialResult[entry.key] = entry.value
+            } else {
+                partialResult[entry.key] = entry.key.oldestSupportedVersion
+            }
+        }
+
         do {
             // One platform with an override.
             let manifest = Manifest.createRootManifest(
@@ -2226,7 +2234,7 @@ class PackageGraphTests: XCTestCase {
                 result.checkTarget("test") { target in
                     var expected = expectedDerivedPlatforms
                     [PackageModel.Platform.macOS, .iOS, .tvOS, .watchOS].forEach {
-                        expected[$0.name] = customXCTestMinimumDeploymentTargets[$0]?.versionString
+                        expected[$0.name] = expectedPlatformsForTests[$0]?.versionString
                     }
                     target.checkDerivedPlatforms(expected)
                     target.checkDerivedPlatformOptions(.macOS, options: ["option1"])
@@ -2254,7 +2262,7 @@ class PackageGraphTests: XCTestCase {
                 result.checkProduct("multi-target") { product in
                     var expected = expectedDerivedPlatforms
                     [PackageModel.Platform.macOS, .iOS, .tvOS, .watchOS].forEach {
-                        expected[$0.name] = customXCTestMinimumDeploymentTargets[$0]?.versionString
+                        expected[$0.name] = expectedPlatformsForTests[$0]?.versionString
                     }
                     product.checkDerivedPlatforms(expected)
                     product.checkDerivedPlatformOptions(.macOS, options: ["option1"])
@@ -2358,7 +2366,7 @@ class PackageGraphTests: XCTestCase {
                 ]
 
                 var expectedDerivedPlatforms = defaultDerivedPlatforms.merging(expectedDeclaredPlatforms, uniquingKeysWith: { lhs, rhs in rhs })
-                var expectedDerivedPlatformsForTests = defaultDerivedPlatforms.merging(customXCTestMinimumDeploymentTargets.map { ($0.name, $1.versionString) }, uniquingKeysWith: { lhs, rhs in rhs })
+                var expectedDerivedPlatformsForTests = defaultDerivedPlatforms.merging(expectedPlatformsForTests.map { ($0.name, $1.versionString) }, uniquingKeysWith: { lhs, rhs in rhs })
                 expectedDerivedPlatformsForTests["ios"] = expectedDeclaredPlatforms["ios"]
 
                 // Gets derived to be the same as the declared iOS deployment target.