I guess it's your implementation's problem. I faced the same but changed something in data structure and now it works. I'm sure it's because function composition and matrix multiplication are non-commutative so the order of evalutation should be preserved.

I didn't have problems with min though.

I tried your solution and I get:
"Expected: equal to 22
Actual: 16".
So writing "...if no output to cout or cerr is getting actually written in test reports" is wrong.
What more do you want?

BTW you fail the first test "dotest(10, 22);". In this case you have the input, you have "actual", you have "expected" and, in addition, you have the corresponding exemple in the description: "Ex: u = [1, 3, 4, 7, 9, 10, 13, 15, 19, 21, 22, 27, ...]"... so you have everything you need to debug your code.

The tests in c++ version are written horribly. How should I debug my solution, if no output to cout or cerr is getting actually written in test reports?
I mean, I can debug it in my terminal and vim, but that kinda defeats the whole purpose of the web-site. Pls, fix

Is c++ version broken? Function composition throws bad_function_call, and matrix_mult gives expected: [unsupported_type, ...] and so is the answer.
Max test works, while min fails. I wonder, why are so many any_cast's in tests?

There exist many comments amounting to the same message as yours.
They are almost all about an incorrect observation. You're going to get an identical answer/reference to existing answers unless you add more information so that it is no longer the same comment as those others.

If you provide a way to reproduce your observation (your code), others can point out the problem. Better for everyone.

I am looking on log that is above my code.
And yes, I've read comments for more than a minute and spent two hours of thinking and coding, so, please, keep your assumptions about my time away.
This was not helpful at all.
Thanks anyways for replying.

That's because you're reading the logs incorrectly. Take 1 minute and have a look at hundred other comments with similar questions.

I don't get it..
Input: a = {2,2,3}, b = {4,9,9}. b is obviously not a squared.
Expected: true.....

While my last version was checking if all elements in b are squares of some number in a, this failed another similar test.

I don't get the point of this exersise.