Reported by Stephan Herrmann:

class A {}
class B extends A { void bar() {} }
public class X {
 public static void main(String[] argv) {
   someWithIndex(getList(), (B b) -> b.bar()); // HERE
 }
 interface ItemWithIndexVisitor<E> {
   public void visit(E item);
 }
 public static <G> void someWithIndex(List<G> list, ItemWithIndexVisitor<G> visitor) {}
 static <I extends A> List<I> getList() { return null; }
}

javac accepts this program, but I don't see how:

During invocation type inference for the marked call I get the following:

The set C contains:
 constraint1: ⟨getList() ⊆throws java.util.List<G#0>⟩
 constraint2: ⟨(B b) -> b.bar() ⊆throws X.ItemWithIndexVisitor<G#0>⟩

Input variables:
 constraint1: none
 constraint2: none

Output variables:
 constraint1: G#0
 constraint2: G#0

No input-output dependencies, so both constraints are picked.

The union of input variables is empty, nothing gets resolved nor substituted
=> constraints remain as given above.

During reduction of constraint2 we fail because of the unsubstituted
inference variable G#0.
This happens at 18.2.5 bullet 3:
 one "of the function type's parameter types is not a proper type" -> FALSE