MethodTypeVars5.java

import org.checkerframework.checker.nullness.qual.*;

class MethodTypeVars5 {
    class B<S extends @Nullable Object> {
        S t;

        B(S t) {
            this.t = t;
        }

        S get() {
            return t;
        }
    }

    B<String> b = new B<String>("Hello World");

    String doit1() {
        return doit1(b);
    }

    <U extends @Nullable Object> U doit1(B<U> x) {
        return x.get();
    }

    String doit2() {
        // Passing the null argument has no effect on the inferred type argument:
        // the second parameter type doesn't contain the type variable at all.
        return doit2(b, null);
    }

    <T extends @Nullable Object> T doit2(B<T> x, @Nullable String y) {
        return x.get();
    }

    String doit3() {
        // Passing the null argument has no effect on the inferred type argument:
        // the type variable only appears as nested type.
        return doit3(null);
    }

    String doit3b() {
        return doit3(new B<String>("Hi"));
    }

    String doit3c() {
        //:: error: (return.type.incompatible)
        return doit3(new B<@Nullable String>("Hi"));
    }

    void doit3d() {
        //:: error: (assignment.type.incompatible)
        @NonNull String s = doit3(new B<@Nullable String>("Hi"));
    }

    void doit3e() {
        String s = doit3(new B<String>("Hi"));
    }

    <T extends @Nullable Object> T doit3(@Nullable B<T> x) {
        if (x != null) {
            return x.get();
        } else {
            // This won't work at runtime, but whatever.
            @SuppressWarnings("unchecked")
            T res = (T) new Object();
            return res;
        }
    }

    String doit4() {
        // Passing the null argument has an impact on the inferred type argument:
        // the type variable appears as the top-level type.
        //:: error: (return.type.incompatible)
        return doit4("Ha!", null);
    }

    <T extends @Nullable Object> T doit4(T x, T y) {
        return x;
    }
}