byte[] generateSyntheticClass(String name);

    /**
     * Injects all the interfaces identified by {@link LegacyTypesSupport#getSyntheticClasses()}
     * into the given classloader.
     */
    void injectEmptyInterfacesIntoClassLoader(ClassLoader classLoader);

    }

    /**
     * Injects the interfaces into an arbitrary classloader via
     * {@link ClassLoader#defineClass(String, byte[], int, int)}.
     */
    @Override
    public void injectEmptyInterfacesIntoClassLoader(ClassLoader classLoader) {
        try {
            for (String name : syntheticClasses) {
                byte[] bytes = generateSyntheticClass(name);

            // Thus, we inject the missing interfaces directly into the system class loader used to load all classes in the test.
            globalServices.get(LegacyTypesSupport.class).injectEmptyInterfacesIntoClassLoader(ProjectBuilderImpl.class.getClassLoader());
        }
        return globalServices;
    }

    private static ServiceRegistry createGlobalServices() {
        return ServiceRegistryBuilder

    ...

// Thrown by ProjectBuilder
org.gradle.api.GradleException: Could not inject synthetic classes.
	at org.gradle.initialization.DefaultLegacyTypesSupport.injectEmptyInterfacesIntoClassLoader(DefaultLegacyTypesSupport.java:91)
	at org.gradle.testfixtures.internal.ProjectBuilderImpl.getGlobalServices(ProjectBuilderImpl.java:182)