Fix linking error on linux

- libstdc++ needs to be added when linking if any dependency has a cpp
  dependency

Author: daveinglis

Sources/SwiftBuildSupport/PackagePIFBuilder+Helpers.swift

@@ -796,6 +796,14 @@ extension PackageGraph.ResolvedProduct {
         self.modules.anySatisfy { !$0.isBinary }
     }
 
+    var hasCXXModules: Bool {
+        let ret = (try? self.recursiveModuleDependencies())?.contains {
+            guard let clangModule = $0.underlying as? ClangModule else { return false }
+            return clangModule.isCXX
+        } ?? false
+        return ret
+    }
+
     /// Returns the corresponding *system library* module, if this is a system library product.
     var systemModule: SystemLibraryModule? {
         guard self.isSystemLibraryProduct else { return nil }

Sources/SwiftBuildSupport/PackagePIFProjectBuilder+Modules.swift

@@ -495,16 +495,6 @@ extension PackagePIFProjectBuilder {
         if enableDuplicateLinkageCulling {
             impartedSettings[.LD_WARN_DUPLICATE_LIBRARIES] = "NO"
         }
-        if sourceModule.isCxx {
-            for platform in ProjectModel.BuildSettings.Platform.allCases {
-                // darwin & freebsd
-                if [.macOS, .macCatalyst, .iOS, .watchOS, .tvOS, .xrOS, .driverKit, .freebsd].contains(platform) {
-                    settings[.OTHER_LDFLAGS, platform] = ["-lc++", "$(inherited)"]
-                } else if [.android, .linux, .wasi, .openbsd].contains(platform) {
-                    settings[.OTHER_LDFLAGS, platform] = ["-lstdc++", "$(inherited)"]
-                }
-            }
-        }
         // This should be only for dynamic targets, but that isn't possible today.
         // Improvement is tracked by rdar://77403529 (Only impart `PackageFrameworks` search paths to clients of dynamic
         // package targets and products).

Sources/SwiftBuildSupport/PackagePIFProjectBuilder+Products.swift

@@ -329,6 +329,7 @@ extension PackagePIFProjectBuilder {
             }
         }
 
+        var needsCppLib: Bool = false
         // Handle the main target's dependencies (and link against them).
         mainModule.recursivelyTraverseDependencies { dependency in
             switch dependency {
@@ -453,6 +454,8 @@ extension PackagePIFProjectBuilder {
                         indent: 1,
                         "Added \(shouldLinkProduct ? "linked " : "")dependency on target '\(dependencyGUID)'"
                     )
+                    needsCppLib = moduleDependency.sources.containsCXXFiles || needsCppLib
+
                 }
 
             case .product(let productDependency, let packageConditions):
@@ -464,6 +467,18 @@ extension PackagePIFProjectBuilder {
                     targetKeyPath: mainModuleTargetKeyPath,
                     settings: &settings
                 )
+                needsCppLib = (productDependency.hasCXXModules || needsCppLib)
+            }
+        }
+
+        if needsCppLib {
+            for platform in ProjectModel.BuildSettings.Platform.allCases {
+                // darwin & freebsd
+                if [.macOS, .macCatalyst, .iOS, .watchOS, .tvOS, .xrOS, .driverKit, .freebsd].contains(platform) {
+                    settings[.OTHER_LDFLAGS, platform] = ["-lc++", "$(inherited)"]
+                } else if [.android, .linux, .wasi, .openbsd].contains(platform) {
+                    settings[.OTHER_LDFLAGS, platform] = ["-lstdc++", "$(inherited)"]
+                }
             }
         }
 

Tests/SwiftBuildSupportTests/PIFBuilderTests.swift

@@ -159,7 +159,7 @@ struct PIFBuilderTests {
         try await withGeneratedPIF(fromFixture: "PIFBuilder/CCPackage") { pif, observabilitySystem in
             let releaseConfig = try pif.workspace
                 .project(named: "CCPackage")
-                .target(id: "PACKAGE-TARGET:CCTarget")
+                .target(id: "PACKAGE-PRODUCT:pifbuilder_ccpackage_executable.executable")
                 .buildConfig(named: "Release")
 
             for platform in ProjectModel.BuildSettings.Platform.allCases {