To reproduce
@Test
    void testModuleSourcePath(Path base) throws Exception {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager fm = compiler.getStandardFileManager(null, null, null);
        Path modules = base.resolve("modules");
        Path classes = Files.createDirectories(base.resolve("classes"));
        tb.writeJavaFiles(modules.resolve("m"), "module m { }", "package p; public class D { }");
        compiler.getTask(null, fm, null,
                Arrays.asList("-modulesourcepath", modules.toString(), "-d", classes.toString()),
                null,
                fm.getJavaFileObjects(modules.resolve("m/p/D.java")))
                .call();

        fm.getLocation(StandardLocation.MODULE_SOURCE_PATH).forEach(System.out::println);
    }

java.lang.UnsupportedOperationException
at com.sun.tools.javac.file.Locations$ModuleSourcePathLocationHandler.getPaths(jdk.compiler@9-internal/Locations.java:1304)
at com.sun.tools.javac.file.Locations.getLocation(jdk.compiler@9-internal/Locations.java:1527)
at com.sun.tools.javac.file.JavacFileManager.getLocation(jdk.compiler@9-internal/JavacFileManager.java:813)