A raw type that is a non-static member type of another raw type (see JLS 4.8) cannot be used with diamond inference. A method reference to such a raw type's constructor should simply use the raw constructor rather than trying to infer a parameterization.

Example:

class Outer<T> {
  class Inner1 {}
  class Inner2<U> {}

  Supplier<Outer.Inner1> s1 = Outer.Inner1::new; // ok
  Supplier<Outer.Inner2> s2 = Outer.Inner2::new; // ok
  Supplier<Outer<T>.Inner1> s3 = Outer.Inner1::new; // unchecked conversion
  Supplier<Outer<T>.Inner2<String>> s4 = Outer.Inner2::new; // unchecked conversion
}