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import java.util.Set;
 
public class Main {
 
    interface Foo<T> { }
 
    interface SubFoo<T> extends Foo<T> { }
 
    static class Bar { }
 
    public static void main(String[] args) {
 
        Set<Foo<?>> arg1 = null;
        Set<SubFoo<?>> arg2 = null;
        Set<Foo<Bar>> arg3 = null;
        Set<SubFoo<Bar>> arg4 = null;
 
        Set<Foo<?>> inflexibleParam;
        inflexibleParam = arg1; //success
        inflexibleParam = arg2; //incompatible types
        inflexibleParam = arg3; //incompatible types
        inflexibleParam = arg4; //incompatible types
 
        Set<? extends Foo<?>> flexibleParam;
        flexibleParam = arg1; //success
        flexibleParam = arg2; //success
        flexibleParam = arg3; //success
        flexibleParam = arg4; //success
    }
}

Main.java:21: error: incompatible types
        inflexibleParam = arg2; //incompatible types
                          ^
  required: Set<Foo<?>>
  found:    Set<SubFoo<?>>
Main.java:22: error: incompatible types
        inflexibleParam = arg3; //incompatible types
                          ^
  required: Set<Foo<?>>
  found:    Set<Foo<Bar>>
Main.java:23: error: incompatible types
        inflexibleParam = arg4; //incompatible types
                          ^
  required: Set<Foo<?>>
  found:    Set<SubFoo<Bar>>
3 errors

Standard output is empty