package lambdatests;

public class UnableToDeriveReturnType {

    void test4() {
        foo((paramToCheck) -> {
            if (paramToCheck != null) {
                return true;
            } else {
                return "String";
            }
        }, "b");
    }

    <T, R> void foo(PossibleFunctionalInterface<T, R> pfi, T paramToCheck) {
        R res = pfi.doSomething(paramToCheck);
        System.out.println(res.toString());
    }

    interface PossibleFunctionalInterface<T, R> {
        R doSomething(T paramToCheck);
    }
}

Use anonymous inner class applied on the lambda specified in method test4 creates code:
    void test4() {
        foo(new PossibleFunctionalInterface<String, >() {

            public doSomething(String paramToCheck) {
                if (paramToCheck != null) {
                    return true;
                } else {
                    return "String";
                }
            }
        }, "b");
    }

which is not compilable.

I would expect that in this case the resulting code should be:
    void test4() {
        foo(new PossibleFunctionalInterface<String, Object>() {

            public Object doSomething(String paramToCheck) {
                if (paramToCheck != null) {
                    return true;
                } else {
                    return "String";
                }
            }
        }, "b");
    }