Javac diagnostics do not cope well with the type-system. Javac diagnostic system falls short when representing complex JDK 5 types such as intersection types, captured types, type-variables and so on. 

Examples:

1)  ********************

CODE:

class Foo<T extends String> {
  <T extends Integer> void foo(T t) {           
      test(t);
  }
  void test(T t) {}
} 

RESULTS:

Test.java:6: test(T) in Foo<T> cannot be applied to (T)
      test(t);
      ^
1 error

PROBLEM:
The diagnostic is ambiguous (as also reported in 5101303). Please add some info about the site in which the type-variable has been declared.

2) **********************************************

CODE: 

import javax.tools.*;
class Test {
  static void test() {
   JavaCompiler compiler = null;
   compiler.getTask(null, null, null,
                 "Bad Argument", null, null);
} }

RESULTS:

Test.java:6: getTask(java.io.Writer,javax.tools.JavaFileManager,
javax.tools.DiagnosticListener<? super javax.tools.JavaFileObject>,
java.lang.Iterable<java.lang.String>,java.lang.Iterable<java.lang.String>,
java.lang.Iterable<? extends javax.tools.JavaFileObject>) in javax.tools.
JavaCompiler cannot be applied to (<nulltype>,<nulltype>,<nulltype>,
java.lang.String,<nulltype>,<nulltype>)
    compiler.getTask(null,null,null,"Bad Argument",null,null);
             ^
1 error

PROBLEMS

Too verbose. Can be improved by dropping qualified names in favor of simple names.

3) **********************************************************************

CODE:

interface List<E> {}

class Test {
    <T> void merge(List<T> l1, List<T> l2) {}
    void test(List<? extends Test> list) {
	merge(list, list);
    }
}

RESULTS:

Test.java:6: <T>merge(List<T>,List<T>) in Test cannot be applied to (List<capture#173 of ? extends Test>,List<capture#228 of ? extends Test>)
	merge(list, list);
	^
1 error

PROBLEMS

The diagnostic is too unfriendly for a non-generics guru.