It is noticed that MethodHandles.collectArguments does not follow its documentation:
 
https://download.java.net/java/early_access/jdk16/docs/api/java.base/java/lang/invoke/MethodHandles.html#collectArguments(java.lang.invoke.MethodHandle,int,java.lang.invoke.MethodHandle)
 
Specifically, it does not thrown an IllegalArgumentException when "pos is not between 0 and the target's arity".
 
Here is a micro test (MHCollectArgumentsBug.java), which highlights the bug:

    import java.lang.invoke.MethodHandles;
    import java.lang.invoke.MethodHandle;
    import java.lang.invoke.MethodType;
 
    public class MHCollectArgumentsBug {
       public static void main(String args[]) throws Throwable {
          MethodType II_I_type = MethodType.methodType(int.class, int.class, int.class);
          MethodType S_I_type = MethodType.methodType(int.class, String.class);
 
          MethodHandle target = MethodHandles.publicLookup().findStatic(MHCollectArgumentsBug.class, "subtract", II_I_type);
          MethodHandle filter = MethodHandles.publicLookup().findStatic(MHCollectArgumentsBug.class, "argStringReturnInt1", S_I_type);
 
          System.out.println("Invalid pos (2)");
 
          try {
             MethodHandles.collectArguments(target, 2, filter);
          } catch (Throwable x) {
             x.printStackTrace();
          }
       }
 
       public static int subtract(int x, int y) { return x - y; }
       public static int argStringReturnInt1(String s) { return 1; }
    }
 
Current reference implementation behaviour:
 
java.lang.ArrayIndexOutOfBoundsException: Index 2 out of bounds for length 2
at java.base/java.lang.invoke.MethodType.parameterType(MethodType.java:775)
at java.base/java.lang.invoke.MethodHandles.collectArgumentsChecks(MethodHandles.java:5520)
at java.base/java.lang.invoke.MethodHandles.collectArguments(MethodHandles.java:5498)
at MHCollectArgumentsBug.main(MHCollectArgumentsBug.java:19)

 
Expected behaviour:
 
java.lang.IllegalArgumentException: Filter argument index (pos) is not between 0 and target arity ("2")
    at java.base/java.lang.invoke.MethodHandles.collectArguments(MethodHandles.java)