Ensure templated closures' return types factor into type resolution

[mszabo-wikia]

Currently type resolution for templated closures where a template type is shared by the closure's return type and a different parameter, e.g. reduce<Tv, Ta>(Traversable<Tv>, (function(Ta, Tv): Ta), Ta $initial): Ta is narrowed down only to the type of the parameter. This is incorrect if the closure may produce a wider range of return types, e.g.:

function test(vec<int> $items): Base {
    return reduce(
        $items,
        ($accum, $item) ==> {
            if ($item > 0) {
                return new NoGet();          // accumulator can become NoGet...
            }
            return new HasGet($accum->get()); // ...so $accum is not provably HasGet
        },
        new HasGet("init"),
    );
}

So do a second closure analysis if the inferred param type widens.