--- a/langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Modules.java	Fri Jan 20 13:20:42 2017 +0100
+++ b/langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Modules.java	Fri Jan 20 15:32:03 2017 +0100
@@ -978,7 +978,7 @@
 
         @Override
         public void visitRequires(JCRequires tree) {
-            if (tree.directive != null) {
+            if (tree.directive != null && allModules().contains(tree.directive.module)) {
                 chk.checkDeprecated(tree.moduleName.pos(), msym, tree.directive.module);
                 msym.directives = msym.directives.prepend(tree.directive);
             }
@@ -1263,7 +1263,6 @@
         msym.requires = msym.requires.appendList(List.from(addReads.getOrDefault(msym, Collections.emptySet())));
 
         List<RequiresDirective> requires = msym.requires;
-        List<RequiresDirective> previous = null;
 
         while (requires.nonEmpty()) {
             if (!allModules().contains(requires.head.module)) {
@@ -1278,13 +1277,7 @@
                 } else {
                     Assert.check((msym.flags() & Flags.AUTOMATIC_MODULE) == 0);
                 }
-                if (previous != null) {
-                    previous.tail = requires.tail;
-                } else {
-                    msym.requires.tail = requires.tail;
-                }
-            } else {
-                previous = requires;
+                msym.requires = List.filter(msym.requires, requires.head);
             }
             requires = requires.tail;
         }

--- a/langtools/test/tools/javac/modules/EdgeCases.java	Fri Jan 20 13:20:42 2017 +0100
+++ b/langtools/test/tools/javac/modules/EdgeCases.java	Fri Jan 20 15:32:03 2017 +0100
@@ -23,13 +23,14 @@
 
 /*
  * @test
- * @bug 8154283 8167320 8171098
+ * @bug 8154283 8167320 8171098 8172809
  * @summary tests for multi-module mode compilation
  * @library /tools/lib
  * @modules
  *      jdk.compiler/com.sun.tools.javac.api
  *      jdk.compiler/com.sun.tools.javac.code
  *      jdk.compiler/com.sun.tools.javac.main
+ *      jdk.compiler/com.sun.tools.javac.util
  * @build toolbox.ToolBox toolbox.JarTask toolbox.JavacTask ModuleTestBase
  * @run main EdgeCases
  */
@@ -44,7 +45,16 @@
 import java.util.Objects;
 import java.util.Set;
 
+import javax.annotation.processing.AbstractProcessor;
+import javax.annotation.processing.RoundEnvironment;
+import javax.annotation.processing.SupportedAnnotationTypes;
+import javax.annotation.processing.SupportedOptions;
+import javax.lang.model.SourceVersion;
 import javax.lang.model.element.Element;
+import javax.lang.model.element.ModuleElement;
+import javax.lang.model.element.ModuleElement.RequiresDirective;
+import javax.lang.model.element.TypeElement;
+import javax.lang.model.util.ElementFilter;
 import javax.tools.JavaCompiler;
 import javax.tools.JavaFileObject;
 import javax.tools.StandardJavaFileManager;
@@ -486,6 +496,85 @@
     }
 
     @Test
+    public void testStripUnknownRequired(Path base) throws Exception {
+        Path src = base.resolve("src");
+        Path src_m1 = src.resolve("m1x");
+        tb.writeJavaFiles(src_m1,
+                          "module m1x { }");
+        Path src_m2 = src.resolve("m2x");
+        tb.writeJavaFiles(src_m2,
+                          "module m2x { }");
+        Path src_m3 = src.resolve("m3x");
+        tb.writeJavaFiles(src_m3,
+                          "module m3x { }");
+        Path src_m4 = src.resolve("m4x");
+        tb.writeJavaFiles(src_m4,
+                          "module m4x { }");
+        Path src_test = src.resolve("test");
+        tb.writeJavaFiles(src_test,
+                          "module test { requires m1x; requires m2x; requires java.base; requires m3x; requires m4x; }");
+        Path src_compile = src.resolve("compile");
+        tb.writeJavaFiles(src_compile,
+                          "module compile { exports p to test; }",
+                          "package p; public class Test { }");
+        Path classes = base.resolve("classes");
+        tb.createDirectories(classes);
+
+        List<String> log = new JavacTask(tb)
+                .options("-processor", ListRequires.class.getName(),
+                         "--module-source-path", src.toString(),
+                         "--limit-modules", "compile",
+                         "-XDaccessInternalAPI=true")
+                .outdir(classes)
+                .files(findJavaFiles(src_compile))
+                .run(Expect.FAIL)
+                .writeAll()
+                .getOutputLines(Task.OutputKind.STDOUT);
+
+        List<String> expected = Arrays.asList(
+                "from directives:",
+                "java.base",
+                "from requires:",
+                "java.base"
+        );
+        if (!Objects.equals(log, expected))
+            throw new AssertionError("Unexpected output: " + log);
+    }
+
+    @SupportedAnnotationTypes("*")
+    @SupportedOptions("expectedEnclosedElements")
+    public static final class ListRequires extends AbstractProcessor {
+
+        private int round;
+
+        @Override
+        public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
+            if (round++ == 0) {
+                ModuleElement compileE = processingEnv.getElementUtils().getModuleElement("compile");
+                ModuleElement testE = ElementFilter.exportsIn(compileE.getDirectives()).get(0).getTargetModules().get(0);
+
+                System.out.println("from directives:");
+                for (RequiresDirective rd : ElementFilter.requiresIn(testE.getDirectives())) {
+                    System.out.println(rd.getDependency().getSimpleName());
+                }
+
+                System.out.println("from requires:");
+                for (RequiresDirective rd : ((ModuleSymbol) testE).requires) {
+                    System.out.println(rd.getDependency().getSimpleName());
+                }
+            }
+
+            return false;
+        }
+
+        @Override
+        public SourceVersion getSupportedSourceVersion() {
+            return SourceVersion.latest();
+        }
+
+    }
+
+    @Test
     public void testOnDemandCompletionModuleInfoJava(Path base) throws Exception {
         Path src = base.resolve("src");
         Path src_m1 = src.resolve("m1x");