But maybe having friends who smoke weed dramatically reduces your ability to assess your friends' assessment of their own cognitive ability.

Indeed, touché

The study is really broken from the beginning:

ask someone who take drugs to assess their own capacities, instead of measuring those from an objective perspective.

Obviously, "yes cannabis affects your ability to self-report your cognitive abilities";

it's a psychoactive drug that is mainly used for its cognitive effects, it's the main use/"advantage" of it ! (more rarely for pain management)

It's like if you ask alkaloidal cocaine (crack) users how they feel about their abilities.

They may report amazing bodily control enhancement, and seen from the outside it may be very different.

Even alcohol users, at the exit of night clubs, you sometimes see them report self-report increased abilities and decide to drive.

In that case, asking them to self-report doesn't give you much info on their actual abilities.

It's pretty well established that using cannabis while the brain is still developing has a negative impact on brain development. Additionally, from what I've observed among a few dozen people I've known over the years, people who start smoking in high school and continue without any substantial breaks through to adulthood tend to have a less healthy relationship with cannabis. They have a stronger tendency to fall into the pattern of using all day every day, and tend use much larger amounts than people who started using as adults or have taken substantial breaks (year +) from cannabis consumption as adults.

You most probably have absolutely 0 clue. I presume you judge them when they are around +-high, or some short time afterwards, or they are rock bottom fails which would be slacking around smoking meth/drinking beers all day if not smoking weed.

Let's have them be clean for a month or two and then do some cognitive assessments that could still be biased (microscopic testing sample), but at least could give some useful info.

Another point where folks (even scientists) often do a basic reasoning mistake - life has to be extremes, you either smoke 0 never ever or are high every day, often all the day yet reality is mostly in between.

Case point 1 - I've done the best, most important decisions of my life when high. Additional perspective was extremely valuable since its a different viewpoint of a person with exactly same experience as you, same dreams, goals, fears. There is literally nobody else like that in whole world and never will be. The connection to subconsciousness becomes a massive highway, you are still only on receiving end but oh boy you get some feedback on various topics from life. But you have to actually listen, hard to do when the goal is only to get wasted and giggle.

Those decisions stood the test of time when looking back even 2 decades. To put some facts into highly opinionated topic - as software dev I've increased my full time 100% classic employment salary from first job till now cca 30x, moved countries 3x (never escaping anything, just curious for challenges and adventures till I found the spot which works for me 100% and raising kids here).

Now I've met folks who when high started seeing blood running down the walls or panic schizophrenic attacks, but these were broken people to begin with. I've seen very similar effects from long term alcohol use (actually life-long schizophrenia triggered by it). So, can we please stop making stupid 1-liner know-it-all statements when clearly topic, and people, are way more complex?