Java 8 type inference

TypeError.java

import java.util.function.Consumer;
import java.util.ArrayList;

public class TypeError {


    public static <A> void let(A a, Consumer<A> f)  {
	f.accept(a);
    }

    public static <A> void newArray(Consumer<ArrayList<A>> f)  {
	f.accept(new ArrayList<A>());
    }

    public static <A> void add(ArrayList<A> ar, A x) {
	ar.add(x);
    }

    public static <A> void addAll(ArrayList<A> ar, ArrayList<A> ar2) {
	ar.addAll(ar2);
    }

    public static <A> void addAll2(ArrayList<A> ar, ArrayList<? extends A> ar2) {
	ar.addAll(ar2);
    }
    
    public static void main(String[] args)  {

	// 0.
	let(new ArrayList<>(), l -> add(l, "abc")); // OK

	// 1.
	let(new ArrayList<String>(), l -> addAll(l, new ArrayList<String>())); // OK

	// 2.
	let(new ArrayList<>(), (ArrayList<String> l) -> addAll(l, new ArrayList<>())); // OK

	// 3.
 	// let(new ArrayList<>(), l -> addAll(l, new ArrayList<String>())); // NG
	
// TypeError.java:25: error: method addAll in class TypeError cannot be applied to given types;
//         let(new ArrayList<>(), l -> addAll(l, new ArrayList<String>())); // NG
//                                     ^
//   required: ArrayList<A>,ArrayList<A>
//   found: ArrayList<Object>,ArrayList<String>
//   reason: inferred type does not conform to equality constraint(s)
//     inferred: String
//     equality constraints(s): String,Object
//   where A is a type-variable:
//     A extends Object declared in method <A>addAll(ArrayList<A>,ArrayList<A>)
// 1 error

	// 4.
 	let(new ArrayList<>(), l -> addAll2(l, new ArrayList<String>())); // OK
	
	let(new ArrayList<String>(), l -> addAll2(l, new ArrayList<String>())); // OK
	let(new ArrayList<>(), (ArrayList<String> l) -> addAll2(l, new ArrayList<>())); // OK
	
    }
    
}