Description Lukas Eder 2021-10-15 09:54:25 EDT

I've declared these three annotations (but any other probably does the trick):

// -------------------------------------
package p;

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

@Target(ElementType.TYPE_USE)
public @interface NotNull {}
// -------------------------------------
package p;

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

@Target(ElementType.TYPE_USE)
public @interface NotNullByDefault {}
// -------------------------------------
package p;

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

@Target(ElementType.TYPE_USE)
public @interface Nullable {}
// -------------------------------------

Now, when I enable annotation based null analysis and violation of null specification, then the following doesn't compile:

// -------------------------------------
package p;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;

public class Test {
    <T> void m1(Function<? super Test, ? extends T> f) {
    }

    void m2() {
        m1(Test::m3);
        m1(t -> t.m3());
    }

    @NotNull List<?> m3() {
        return new ArrayList<>();
    }
}
// -------------------------------------

There's an error on the method reference that doesn't appear on the lambda:

> Null type mismatch at method return type: Method descriptor Function<Test,List<capture#1-of ?>>.apply(Test) promises '@NotNull List<capture#of ?>' but referenced method provides '@NotNull List<?>'

The error disappears when I'm not using wildcards. Both of these work:

// -------------------------------------
package p;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;

public class Test<X> {
    <T> void m1(Function<? super Test<?>, ? extends T> f) {
    }

    void m2() {
        m1(Test::m3);
        m1(t -> t.m3());
    }

    @NotNull List<X> m3() {
        return new ArrayList<>();
    }
}
// -------------------------------------
package p;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;

public class Test {
    <T> void m1(Function<? super Test, ? extends T> f) {
    }

    void m2() {
        m1(Test::m3);
        m1(t -> t.m3());
    }

    <X> @NotNull List<X> m3() {
        return new ArrayList<>();
    }
}
// -------------------------------------