8066746: MHs.explicitCastArguments does incorrect type checks for VarargsCollector

Reviewed-by: jrose, psandoz

src/share/classes/java/lang/invoke/MethodHandles.java

@@ -2029,7 +2029,7 @@ return invoker;
         MethodType oldType = target.type();
         if (oldType == newType)  return target;
         if (oldType.explicitCastEquivalentToAsType(newType)) {
-            return target.asType(newType);
+            return target.asFixedArity().asType(newType);
         }
         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
     }

test/java/lang/invoke/ExplicitCastArgumentsTest.java

@@ -32,15 +32,30 @@ import sun.invoke.util.Wrapper;
  */
 public class ExplicitCastArgumentsTest {
     private static final boolean VERBOSE = Boolean.getBoolean("verbose");
+    private static final Class<?> THIS_CLASS = ExplicitCastArgumentsTest.class;
 
     public static void main(String[] args) throws Throwable {
+        testVarargsCollector();
+        testRef2Prim();
+        System.out.println("TEST PASSED");
+    }
+
+    public static String[] f(String... args) { return args; }
+
+    public static void testVarargsCollector() throws Throwable {
+        MethodType mt = MethodType.methodType(String[].class, String[].class);
+        MethodHandle mh = MethodHandles.publicLookup().findStatic(THIS_CLASS, "f", mt);
+        mh = MethodHandles.explicitCastArguments(mh, MethodType.methodType(Object.class, Object.class));
+        mh.invokeWithArguments((Object)(new String[] {"str1", "str2"}));
+    }
+
+    public static void testRef2Prim() throws Throwable {
         for (Wrapper from : Wrapper.values()) {
             for (Wrapper to : Wrapper.values()) {
                 if (from == Wrapper.VOID || to == Wrapper.VOID) continue;
-                testRef2Prim (from, to);
+                testRef2Prim(from, to);
             }
         }
-        System.out.println("TEST PASSED");
     }
 
     public static void testRef2Prim(Wrapper from, Wrapper to) throws Throwable {