First of all, I've been told that you can use the future simple tense when you promise/intend to do something not later than some date, as in:

"I'll do my homework by 9 pm". 

You can use the future perfect if you confidently predict something: 

"I will have done my homework by 9 pm". 

I haven't seen any explanation for this in Grammar books (which is odd), but, nonetheless, it's what I've been told on this forum.

I have been comparing two resources (The [first][1] and the [second][2]) which seem contradictory:

The first states:

> Don't use certain strict expressions denoting time that require
> certain complicated tenses. For example, "***by the time***" requires the
> perfect tenses. If you replace "by the time" with "before" or "after",
> you can use the simple tenses, especially in everyday speech.
> 
> I will have finished the report by six. I will finish the report
> before six.

The second:

> Sometimes, you can use the future perfect tense **and the simple future
> tense interchangeably**. In these two sentences, there is no real
> difference in meaning because the word *before* makes the sequence of
> events clear:
> 
> Linda will leave before you get there. Linda will have left before you
> get there.
> 
> But without prepositions such as *before* ***or by the time*** that make
> the sequence of events clear, you need to use the future perfect to
> show what happened first.

Here are two sentences I've made that should mean the same and be correct according to the second source:

**A**) Mary will leave by the time you get there.

**B**) Mary will have left by the time you get there.

To recapitulate, according to the second source *"there is no real difference in meaning"* in the sentences **A** and **B** and I *"can use the future perfect tense and the simple future tense interchangeably"* 

Whereas, according to the first source "by the time" **requires** the perfect tenses, that is I cannot use the simple future tense in **A**.

So, which source is correct?


  [1]: https://usefulenglish.ru/grammar/tenses-summary-and-tips-on-usage
  [2]: https://www.grammarly.com/blog/future-perfect/