See https://bugs.eclipse.org/bugs/show_bug.cgi?id=121369

Apparently, the specification requires a compiler must infer
List<T>, not List<Object> for U in a call to Foo.foo() in this
example:

class Foo {
  static <T, U extends java.util.List<T>> void foo() {
    return;
  }
}
A further issue with uninferred type variables:

Can you explain how javac is accepting the following code if not doing what I am suggesting below?
  public <E, S extends java.util.Collection<E>> S test1(S param){ return null; }
  public void test2() { test1(new Vector<String>()); }

15.12.2.7 determines that: S = Vector<String>.
Then 15.12.2.8 kicks in to infer unconstrained E.
  * S >> R' means: Object >> Vector<String>
  * Bi[T1 = B(T1) ... Tn = B(Tn)] >> Ti means: Collection<E> >> S

Stepping back for an instant; what are we doing here? S already got inferred, so why would it try to infer it over again? Where is the fact that S = Vector<String> appearing then?

I think the spec should rather say
  Bi[T1 = B(T1) ... Tn = B(Tn)] >> B(Ti) (note 'B(Ti)' instead of 'Ti')
but then the algorithm described in 15.12.2.7 wouldn't yield anything as:
  Collection<E> >> Vector<String>
doesn't yield any data (as no formal argument on right side).

This is why I would suggest injecting:
  Vector<String> << Collection<E>
If you don't do this, how can you infer E to be String instead of Object?
(E=Object would invalidate the bound contract for S, as Vector<String> is not <: Collection<Object>.)

If you change the line:
  test1(new Vector<String>());
into:
  String s = (String) test1(new Vector<String>());
you can see that javac did infer S to be Vector<String> as per error message tells.

To be clear my spec suggestion is that instead of 15.12.2.8 only saying:
  * additional constraints Bi[T1 = B(T1) ... Tn = B(Tn)] >> Ti,
    where Bi is the declared bound of Ti,
It would also hint from bounds of already inferred type variables, since
these may contain references to underconstrained variables.
  * additional constraints B(Ti) << Bi[T1 = B(T1) ... Tn = B(Tn)],
    where Bi is the declared bound of Ti,