Why is generic of a return type erased when there is an unchecked conversion of a method parameter in Java 8?

This looks like a known compatibility issue reported here and here.

From the second link:

The following code which compiled, with warnings, in JDK 7 will not
compile in JDK 8:

import java.util.List;
class SampleClass {

     static class Baz<T> {
         public static List<Baz<Object>> sampleMethod(Baz<Object> param) {
             return null;
         }
     }

     private static void bar(Baz arg) {
         Baz element = Baz.sampleMethod(arg).get(0);
     }
}

Compiling this code in JDK 8 produces the following error:

SampleClass.java:12: error:incompatible types: Object cannot be
converted to Baz

Baz element = Baz.sampleMethod(arg).get(0);

Note: SampleClass.java uses unchecked or unsafe operations. Note:
Recompile with -Xlint:unchecked for details. 1 error

Deriving from this, the OP’s code can be fixed by replacing this line (the type declartion on the right hand side threw me off – I read it as a typed array list which it is not):

List list = new ArrayList<Integer>();

with

List<Integer> list = new ArrayList<Integer>();

which will not result in type being being erased from return type of method getProducer(List<Integer> list)

Quote from second link again:

In this example, a raw type is being passed to the
sampleMethod(Baz<Object>) method which is applicable by subtyping (see
the JLS, Java SE 7 Edition, section 15.12.2.2). An unchecked
conversion is necessary for the method to be applicable, so its return
type is erased (see the JLS, Java SE 7 Edition, section 15.12.2.6). In
this case the return type of sampleMethod(Baz<Object>) is
java.util.List instead of java.util.List<Baz<Object>> and thus the
return type of get(int) is Object, which is not assignment-compatible
with Baz.