Add group parent membership regression tests

The first two tests reproduce multiple parents: a source pointing at the
spec directory, and a directory reached first through a file source. The
third preserves a skipped subdirectory at the project root when a later
source populates it. `expectSingleParents` lives in TestSupport so the
fixtures are covered too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Ckitakishi

Sources/TestSupport/TestHelpers.swift

@@ -22,6 +22,28 @@ public func unwrap<T>(_ value: T?, file: String = #file, line: Int = #line) thro
     }
 }
 
+public func expectSingleParents(_ pbxProj: PBXProj, function: String = #function, file: String = #file, line: Int = #line) throws {
+    func label(_ element: PBXFileElement) -> String {
+        element.name ?? element.path ?? ""
+    }
+
+    var parentsByElement: [ObjectIdentifier: (element: PBXFileElement, groups: [String])] = [:]
+    let allGroups: [PBXGroup] = pbxProj.groups + pbxProj.variantGroups + pbxProj.versionGroups
+    for group in allGroups {
+        for child in group.children {
+            parentsByElement[ObjectIdentifier(child), default: (child, [])].groups.append(label(group))
+        }
+    }
+    let duplicates = parentsByElement.values.filter { $0.groups.count > 1 }
+    if !duplicates.isEmpty {
+        let description = duplicates
+            .map { "\(label($0.element).quoted) is a child of \($0.groups.map(\.quoted).joined(separator: " and "))" }
+            .sorted()
+            .joined(separator: "\n")
+        throw failure("Elements with more than one parent group:\n\(description)", function: function, file: file, line: line)
+    }
+}
+
 public func expectError<T: Error>(_ expectedError: T, function: String = #function, file: String = #file, line: Int = #line, _ closure: () throws -> Void) throws where T: CustomStringConvertible {
     do {
         try closure()

Tests/FixtureTests/FixtureTests.swift

@@ -30,6 +30,7 @@ private func generateXcodeProject(specPath: Path, file: String = #file, line: In
     let generator = ProjectGenerator(project: project)
     let writer = FileWriter(project: project)
     let xcodeProject = try generator.generateXcodeProject(userName: "someUser")
+    try expectSingleParents(xcodeProject.pbxproj, file: file, line: line)
     try writer.writeXcodeProject(xcodeProject)
     try writer.writePlists()
 }

Tests/XcodeGenKitTests/SourceGeneratorTests.swift

@@ -84,6 +84,81 @@ class SourceGeneratorTests: XCTestCase {
                 try pbxProj.expectFile(paths: ["Sources", "A", "C2.0", "c.swift"], buildPhase: .sources)
             }
 
+            $0.it("generates a single parent for groups when a source is the base path") {
+                let directories = """
+                Module:
+                  - Extension:
+                    - a.swift
+                  - Model:
+                    - b.swift
+                  - c.swift
+                """
+                try createDirectories(directories)
+
+                let target = Target(name: "Module", type: .framework, platform: .iOS, sources: ["../Module"])
+                let project = Project(basePath: directoryPath + "Module", name: "Module", targets: [target])
+
+                let generator = PBXProjGenerator(project: project, projectDirectory: directoryPath)
+                let pbxProj = try generator.generate()
+
+                try pbxProj.expectFile(paths: ["Module", "Extension", "a.swift"], buildPhase: .sources)
+                try pbxProj.expectFile(paths: ["Module", "Model", "b.swift"], buildPhase: .sources)
+                try pbxProj.expectFile(paths: ["Module", "c.swift"], buildPhase: .sources)
+                try expectSingleParents(pbxProj)
+
+                let mainGroupChildren = try pbxProj.getMainGroup().children.map(\.nameOrPath)
+                try expect(mainGroupChildren.contains("Extension")) == false
+                try expect(mainGroupChildren.contains("Model")) == false
+            }
+
+            $0.it("generates a single parent for a group first referenced as a top level file source") {
+                let directories = """
+                Sources:
+                  - Pages:
+                    - a.swift
+                  - b.swift
+                """
+                try createDirectories(directories)
+
+                // The file source creates Pages at the top level before the directory source reparents it.
+                let fileTarget = Target(name: "A", type: .application, platform: .iOS, sources: ["Sources/Pages/a.swift"])
+                let directoryTarget = Target(name: "B", type: .application, platform: .iOS, sources: ["Sources"])
+                let project = Project(basePath: directoryPath, name: "Test", targets: [fileTarget, directoryTarget])
+
+                let pbxProj = try project.generatePbxProj()
+
+                try pbxProj.expectFile(paths: ["Sources", "Pages", "a.swift"], buildPhase: .sources)
+                try pbxProj.expectFile(paths: ["Sources", "b.swift"], buildPhase: .sources)
+                try expectSingleParents(pbxProj)
+
+                let mainGroupChildren = try pbxProj.getMainGroup().children.map(\.nameOrPath)
+                try expect(mainGroupChildren.contains("Sources/Pages")) == false
+            }
+
+            $0.it("keeps a subdirectory reachable when the base source excludes it and another source adds it") {
+                let directories = """
+                Module:
+                  - Resources:
+                    - a.json
+                  - b.swift
+                """
+                try createDirectories(directories)
+
+                // The excluded directory is skipped by its parent, then populated through a later source.
+                let target = Target(name: "Module", type: .framework, platform: .iOS, sources: [
+                    TargetSource(path: "../Module", excludes: ["Resources/**"]),
+                    TargetSource(path: "Resources", buildPhase: .resources),
+                ])
+                let project = Project(basePath: directoryPath + "Module", name: "Module", targets: [target])
+
+                let generator = PBXProjGenerator(project: project, projectDirectory: directoryPath)
+                let pbxProj = try generator.generate()
+
+                try pbxProj.expectFile(paths: ["Module", "b.swift"], buildPhase: .sources)
+                try pbxProj.expectFile(paths: ["Module/Resources", "a.json"], names: ["Resources", "a.json"], buildPhase: .resources)
+                try expectSingleParents(pbxProj)
+            }
+
             $0.it("generates synced folder") {
                 let directories = """
                 Sources: