Total cover states the attack can't happen, and the Roper tendril attack is not an area of effect, so it should not be able to just attack with disadvantage. But...if I was running it, I would have allowed the Roper to attack IF it guessed the proper square your Warlock was in.
Basically, the Roper saw him go around the corner, so is going to try. Let's say that there are 12 squares that are outside the Roper's vision around the corner and within it's reach. I'd number them, and roll a d12. If I rolled the square next to your Warlock, I'd describe it in game that the tendril lashes out and missed, but tell the Warlock he's still in danger. If the next turn I rolled the square the Warlock was in I'd roll an attack for the Roper at disadvantage.
To me, that makes more sense, and adds a bit of excitement vs just not allowing the attack at all. But at the end of the day, it's up to your DM.
By they way combat rules are laid out the Roper could not see the target, thus it cannot attack the target. AoE spells already give the ability to negate total cover by their description, so you also have a method for handling total cover situation if you have the right tools.
If, however, the DM wants to create some reason the creature could make the attack, that's entirely up to the DM. The trade off here is going to be that certain spells and attacks might allow for the player to do the same if they have the reach like the Roper. It gets really tricky since you're giving the players/creatures the ability to negate a powerful bonus when using cover and tactical movement.
Sure, if it's invisible, in a cloud type effect, darkness, or similar. If there is a wall between you and the target, such as this situation, I find it difficult to say that you can attack around a corner at an unseen target.
I can definitely see the case for the the tentacles being able to make a "blind" attack around the corner, seeing as the Roper had vision of the Warlock, who moved around the corner and attacked, before retreating. The Roper perceived all of this, yes?
So the question is how you would handle the inability to track the target past the corner, assuming the attack is able to reach that far. I would say that attacking with disadvantage is not enough.... well... disadvantage. I like the idea of needing some form of accuracy roll. If the attack does actually land on the Warlock, then roll the attack at disadvantage.
Given the use of tentacles, I would say it can attack around cover, but it should have to roll a perception check with disadvantage, and if it succeeds, then it could roll attack with disadvantage, especially given the distance.
Considering that the Roper is unique because it has 50' of reach with it's tentacles, I'd definitely say that it could attack blindly around corners that way. I'd do the attack the way a former DM had us attack opponents who were invisible, I'd have it pick which square it thinks the Warlock is in and then make the attack roll on that square with disadvantage. I love the suggestion of rolling randomly to determine which square it picks too because that's the only way that is fair to me since the DM knows which square the Warlock is in.
But, Ropers aren't very intelligent. I'm not sure that a Roper would be smart enough to try that when other PC's are visible instead of being around the corner and are in reach. Reaching around the corner like that is something that smarter monsters would do and Ropers are pretty stupid and they don't speak any languages. They're smarter than animals, but they're not Goblins!
I would allow a roper to attack blindly around a corner with its tentacles. I would definitely roll the attack with disadvantage as it is an attack on an unseen opponent. I would probably also roll randomly to determine exactly where the roper tried to attack. The blindly groping tentacles might thus attack the warlock, might attack somebody else in the party, or might attack empty spaces.
If the roper had foes between it and the corner that the warlock ducked around I would have it attack the visible foes in preference to a foe that ducked back around the corner.
The rules don't allow melee attacking a target through full cover. This is usually not the case for PC's attacking around corners, unless they are using a flexible weapon (in which case, I might allow for a single, rule-breaking instance).
The only times I've seen tentacles attack while the attached monsters was not in view (example: Kraken or huge squid tentacles attacking a ship), those tentacles were stat'ed as separate creatures, allowing PC's to attack (and 'kill') them.
fwiw: semantically, the current Roper description calls them 'tendrils' rather than 'tentacles'. Not sure what that means, but I equate that with being more rope-like than normal tentacles, and thus seem less likely to attack around corners (but that's merely how I envision them).
The Roper grabs with the tendrils, so wouldn't that imply a level of prehensileness?
Depends: normal (i.e. plant) tendrils grasp without being prehensile, and the 3e Roper grasped via non-prehensile "strands". I dunno why the 5e roper says "tendrils" rather than "tentacles", but I'm gonna treat 'em different, only 'cuz I'm pedantic that way. It merits discussion though (especially when you factor in 1e "tentacle" ropers).
Remember the rules are designed to cover majority of situations not every edge case. I think we can agree "Roper in a 20' hall with a 90' bend" is an edge case.
To the same extent I could grab something I can't see if I stuck my arm through a hole and fumbled for it. It could also be argued most things I grab for like that aren't fighting back. Anything I try to reach for without vision isn't animate much less able to fight back.
Options are simple: 1) The Roper can't make attacks against things it can't see because it's how the rules are written and surprising players in a combat with house rules is a bad generally bad form. 2) The Roper can't make attacks against things it can't see because it's how the rules are written and it would be difficult for the Roper to effectively grab or attack things that can actively defend itself. 3) The Roper can't make attacks against things it can't see because it's how the rules are written, but Rule of Cool and this "monster movie" scene and would enhance the story so the Roper can make attacks at DisAdvantage. The GM makes it clear this is a very specific "house ruling" in this instance to make this combat more dramatic.
Tendril: "a slender threadlike appendage of a climbing plant, often growing in a spiral form, that stretches out and twines around any suitable support."
Tentacle: "a slender, flexible limb or appendage in an animal, especially around the mouth of an invertebrate, used for grasping or moving about, or bearing sense organs.
(in a plant) a tendril or a sensitive glandular hair.
something resembling a tentacle in shape or flexibility."