8255531: MethodHandles::permuteArguments throws NPE when duplicating dropped arguments

Reviewed-by: redestad

Author: JornVernee

src/java.base/share/classes/java/lang/invoke/LambdaFormBuffer.java

@@ -301,7 +301,7 @@ LambdaForm endEdit() {
             int argp = firstChange, exprp = 0;
             for (int i = firstChange; i < arity; i++) {
                 Name name = names[i];
-                if (name.isParam()) {
+                if (name != null && name.isParam()) {
                     names[argp++] = name;
                 } else {
                     exprs[exprp++] = name;

test/jdk/java/lang/invoke/MethodHandlesPermuteArgumentsTest.java

@@ -63,6 +63,8 @@ public void testPermuteArguments0() throws Throwable {
         testBadReorderIndex();
         testReturnTypeMismatch();
         testReorderTypeMismatch();
+
+        testPermuteWithEmpty();
     }
 
     public void testPermuteArguments(int max, Class<?> type1, int t2c, Class<?> type2, int dilution) throws Throwable {
@@ -227,6 +229,12 @@ public void testReorderTypeMismatch() throws Throwable {
                 IllegalArgumentException.class, ".*parameter types do not match after reorder.*");
     }
 
+    // for JDK-8255531
+    private void testPermuteWithEmpty() {
+        MethodHandle mh = MethodHandles.empty(MethodType.methodType(void.class, int.class, int.class));
+        MethodHandles.permuteArguments(mh, MethodType.methodType(void.class, int.class), 0, 0);
+    }
+
     private interface RunnableX {
         void run() throws Throwable;
     }