Note the default linkage of a Swift library is shared linkage as shown in the diagram.

.Swift Library Plugin default task graph
image::swift-shared-library-task-graph.png[]

With static linkage, the diagram changes to the following:

.Swift Library Plugin static library only task graph
image::swift-static-library-task-graph.png[]

[[sec:swift_library_task_variants]]
=== Variant-dependent Tasks

                                    lib project: ':api', library: 'api'
                                    lib project: ':spi', library: 'spi'
                                    lib project: ':runtime', library: 'runtime'
                                    lib library: 'bootstrap'
                                    lib project: ':libraries', library: 'foo'
                                    lib project: ':libraries', library: 'bar'

        final DefaultSwiftLibrary library = componentFactory.newInstance(SwiftLibrary.class, DefaultSwiftLibrary.class, "main");
        project.getExtensions().add(SwiftLibrary.class, "library", library);
        project.getComponents().add(library);

        // Setup component
        final Property<String> module = library.getModule();
        module.set(GUtil.toCamelCase(project.getName()));

Note the default linkage of a {cpp} library is shared linkage as shown in the diagram.

.{cpp} Library Plugin default task graph
image::cpp-shared-library-task-graph.png[]

With static linkage, the diagram changes to the following:

.{cpp} Library Plugin static library only task graph
image::cpp-static-library-task-graph.png[]

[[sec:cpp_library_task_variants]]
=== Variant-dependent Tasks

                apply plugin: 'cpp-library'
                dependencies {
                    api project(':lib2')
                    implementation project(':lib3')
                }
            }
            project(':lib2') {
                apply plugin: 'cpp-library'
            }
            project(':lib3') {
                apply plugin: 'cpp-library'
            }
"""

        "does not exist"                       | "library: 'unknown'"                    | "Could not locate library 'unknown' required by 'main' in project ':exe'."
        "project that does not exist"          | "project: ':unknown', library: 'hello'" | "Project with path ':unknown' not found."

                task.getTargetPlatform().set(library.getNativePlatform());
                task.getToolChain().set(library.getToolChain());
            });

            // Wire the task into the library model
            library.getLinkFile().set(createTask.flatMap(task -> task.getBinaryFile()));
            library.getLinkFileProducer().set(createTask);
            library.getCreateTask().set(createTask);

    }

    def "fails when no linkage is specified"() {
        def library = new SwiftLib()
        buildFile << """
            apply plugin: 'swift-library'

            library {
                linkage = []
            }
        """
        settingsFile << """
            rootProject.name = 'foo'
        """
        library.writeToProject(testDirectory)

        when:
        fails('assemble')

            sources {
                cpp.lib library: 'hello', linkage: 'static'
            }
        }
    }
}
"""

        when:
        fails "mainExecutable"

        then:
        failure.assertHasDescription("Execution failed for task ':linkMainExecutable'.")

            project(':cppGreeter') {
                apply plugin: 'cpp-library'
                dependencies {
                    implementation project(':logger')
                }
                library.linkage = [Linkage.${linkage}]
            }
            project(':logger') {
                apply plugin: 'cpp-library'
                library.linkage = [Linkage.${linkage}]
            }
        """