Add Cpp support

Sources/Build/Command.compile(ClangModule).swift

@@ -105,6 +105,7 @@ extension Command {
         var args = module.basicArgs
         args += module.optimizationFlags(conf)
         args += ["-L\(prefix)"]
+        args += module.languageLinkArgs
         args += module.linkFlags
         args += module.sources.compilePathsForBuildDir(wd).map{$0.object}
         args += Xld

Sources/Build/misc.swift

@@ -43,6 +43,45 @@ extension CModule {
     }
 }
 
+extension String {
+    var isCpp: Bool {
+        guard let ext = self.fileExt else {
+            return false
+        }
+        return Sources.validCppExtensions.contains(ext)
+    }
+}
+
+extension ClangModule {
+    // Returns language specific arguments for a ClangModule.
+    var languageLinkArgs: [String] {
+        var args = [String]() 
+        // Check if this module contains any cpp file.
+        var linkCpp = self.containsCppFiles
+
+        // Otherwise check if any of its dependencies contains a cpp file.
+        // FIXME: It is expensive to iterate over all of the dependencies.
+        // Figure out a way to cache this kind of lookups.
+        if !linkCpp {
+            for case let dep as ClangModule in recursiveDependencies {
+                if dep.containsCppFiles {
+                    linkCpp = true
+                    break
+                }
+            }
+        }
+        // Link C++ if found any cpp source. 
+        if linkCpp {
+            args += ["-lc++"]
+        }
+        return args
+    }
+
+    var containsCppFiles: Bool {
+        return sources.paths.contains { $0.isCpp } 
+    }
+}
+
 extension ClangModule {
 
     public enum ModuleMapError: ErrorProtocol {

Sources/PackageLoading/PackageExtensions.swift

@@ -151,7 +151,7 @@ extension Package {
     private func modulify(_ path: String, name: String) throws -> Module {
         let walked = walk(path, recursing: shouldConsiderDirectory).map{ $0 }
 
-        let cSources = walked.filter{ isValidSource($0, validExtensions: Sources.validCExtensions) }
+        let cSources = walked.filter{ isValidSource($0, validExtensions: Sources.validCFamilyExtensions) }
         let swiftSources = walked.filter{ isValidSource($0, validExtensions: Sources.validSwiftExtensions) }
 
         if !cSources.isEmpty {

Sources/PackageModel/Sources.swift

@@ -30,6 +30,10 @@ public struct Sources {
     static public var validSwiftExtensions = Set<String>(["swift"])
 
     static public var validCExtensions = Set<String>(["c", "m"])
-
-    static public var validExtensions = { validSwiftExtensions.union(validCExtensions) }()
+
+    static public var validCppExtensions = Set<String>(["mm", "cc", "cpp", "cxx"])
+
+    static public var validCFamilyExtensions = validCExtensions.union(validCppExtensions)
+
+    static public var validExtensions = { validSwiftExtensions.union(validCExtensions).union(validCFamilyExtensions) }()
 }