JSpecify: handle return types of method references in Java Generics

[akulk022]

Adding new test case and the logic to handle the negative scenario of the test case where we were getting a false positive earlier.

class Test {
    interface A<T1 extends @Nullable Object> {
        T1 function(Object o);
    }
    static @Nullable String foo(Object o) {
        return o.toString();
    }
    static void testPositive() {
    // BUG: Diagnostic contains: referenced method returns @Nullable
	A<String> p = Test::foo;
    }
    static void testNegative() {
    // We were getting a false positive error on this line as the Nullability of the overriden method was not identified
        A<@Nullable String> p = Test::foo;
    }
}

All test cases in NullAwayJSpecifyGenericTests.java passed for these changes.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (099a7a5) 86.86% compared to head (56f95e3) 86.87%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #847   +/-   ##
=========================================
  Coverage     86.86%   86.87%           
- Complexity     1879     1880    +1     
=========================================
  Files            74       74           
  Lines          6196     6200    +4     
  Branches       1202     1202           
=========================================
+ Hits           5382     5386    +4     
  Misses          406      406           
  Partials        408      408           

Files	Coverage Δ
...rc/main/java/com/uber/nullaway/GenericsChecks.java	90.28% <100.00%> (ø)
...away/src/main/java/com/uber/nullaway/NullAway.java	89.77% <100.00%> (+0.03%)	⬆️

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[msridhar]

Great start! Couple comments

[msridhar]

As noted in an offline discussion, this test looks good but let's add tests for more (pseudo-)assignments, like passing a method reference as a function parameter and returning a method reference

[msridhar]

LGTM!