Assert include directory exists in Clang target

Author: elsh

Sources/PackageModel/Target.swift

@@ -10,6 +10,7 @@
 
 import TSCBasic
 import TSCUtility
+import Foundation
 
 public class Target: ObjectIdentifierProtocol, PolymorphicCodableProtocol {
     public static var implementations: [PolymorphicCodableProtocol.Type] = [
@@ -427,7 +428,8 @@ public final class ClangTarget: Target {
         dependencies: [Target.Dependency] = [],
         buildSettings: BuildSettings.AssignmentTable = .init()
     ) {
-        assert(includeDir.contains(sources.root), "\(includeDir) should be contained in the source root \(sources.root)")
+        
+        assert(includeDir.isDirectory && includeDir.contains(sources.root), "\(includeDir) should be contained in the source root \(sources.root)")
         self.isCXX = sources.containsCXXFiles
         self.cLanguageStandard = cLanguageStandard
         self.cxxLanguageStandard = cxxLanguageStandard
@@ -666,3 +668,12 @@ public enum PluginCapability: Equatable, Codable {
         }
     }
 }
+
+
+fileprivate extension AbsolutePath {
+  var isDirectory: Bool {
+      var isDir: ObjCBool = false
+      let exists = FileManager.default.fileExists(atPath: self.pathString, isDirectory: &isDir)
+      return exists && isDir.boolValue
+  }
+}