The question is not whether the warlock can attack with the Polearm or not. Your whole answer is based around that. The AoO is triggered, whether you are able to attack or not. Did you read the Warcaster Feat? The last part of your answer looks like you didn't. It allows you to perform somatic components while wearing weapons and/or a shield.

@Thyzer you didn't read the answer correctly then: the problem is to get a shield AND have the AoO from a polearm. Because you need to be able to attack to benefit from the reach to get the AoO. To be able to attack you need two hands, so you can't hold a shield in one hand. You need a third hand for the shield

What let's you believe that for the enemy to provoke an opportunity attack you need to be able to attack? Warcaster let's you replace this attack of opportunity with a spell. So even though you are unable to perform an attack you can replace the possibility of an attack with a spell. The enemy provokes an AoO whether you can attack or not, AFAIK.

Wielding a two handed weapon requires only 1 hand. The two handed trait explicitly says that you only need two hands when you attack.

@Thyzer To benefit from a weapons properties you need to properly use it. Reach is a property of the helbard. The Helbard requires both hands. Enemies provoke an AoO if they leave a threatened square. You can't threaten a squeare if you don't use the weapon properly.

Without the reach property the reach would be 5ft. and you could cast a spell as AoO then, even with shield. Are we agreeing on that?

@Thyzer yes, but you'd be at disadvantage for casting in a field directly next to an enemy.

@Trish that would answer the question with a "yes, but...". I am currently looking through some rule texts and sage advice.

I agree with you, that realistically this is really stupid, but OP wanted a RAW answer

My whole answer is based on one thing: shield + Helbard. And THAT is a NONO

unless you have a third hand

Ok, lets break it up... Warcaster lets you replace any AoO provoked by enemy movement with a spell (action, single target, ...), and you can cast spells when wielding a shield/weapon. Polearm Master makes that enemy provoke an AoO when moving in your reach. - without shield that would mean an enemy moving within 10ft of you, gets an eldritch blast to the face. Thats clear... With shield you would still be able to cast a spell.

So the point where we disagree is that the AoO is still provoked when you are unable to perform an attack

The PHB chapter says "You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach."

the opportunity attack is provoked whether you use it or not. Restrictions seem to be: "You can avoid provoking an opportunity attack by taking the Disengage action. You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy."

Looking for the rule about the properties...

It has to be a melee attack, but we replace that with a spell nonetheless. And it triggers, in case of Polearm Master, when they enter the reach while wielding a glaive, halberd, pike or quarterstaff. (Wielding does not require 2 hands, only attacking does)

The problem is reach

I'm failing to find the rule that you can't benefit from reach if you cant attack

Do you get reach from a weapon you don't qualify to use?

"Reach. This weapon adds 5 feet to your reach when you attack with it, as well as when determining your reach for opportunity attacks with it (see chapter 9)."

I am convinced that you need to be able to attack with a weapon to benefit from its properties.

at least there is no specific rule

it has to be a general then

"when you attack with it" => You need to attack with it. A Helbard has "This weapon requires two hands when you attack with it"

=> without 2 hands on the helbard, you don't get reach.

"as well as when determining your reach for opportuntiy attacks"

Without reach, you don't get the AoO at 10 feet range

You need to qualify for the weapon fully to get the other traits of the weapon.

If you use one hand on a helbard, you don't properly use it, and thus shouldn'T benefit from reach.

The way out I searched is simple: get the shield into a non-hand-limb

to have 2 hands that are allowed to wield weapons.

@Thyzer also, clean up after starting a chat discussion.

there is a magic item for a floating shield :P ... "You need to qualify for the weapon fully to get the other traits of the weapon." <- is that a rules text?

not strictly, but common sense.

in RAW questions common sense doesn't count

it is not a RAW question

by common sense I could attack with a halberd while holding it in only one hand... i probably would miss, counts as attack though.

well, the title contains RAW... someone should add that tag

it needs the rules-as-written tag to count.

yes, RAW is a mandatory tag.

if it shall be RAW

@Thyzer 5 tag maximum :P

as long as the RAW tag is not there, RAW is not enforced.

@Thyzer no, by logic not. I own a helbard.... you need two hands to do anything but march with it

I think it is clear that OP intended it to be a RAW question. But even if it isn't tagged as such, I may answer under a RAW stand point, unless you can show me where it says that you cannot use the reach property I will stay with my answer

regarding your answer, well i don't know :P

I would still allow it ingame, but thats not the point

@Thyzer intention and tagging are not the same.

tag trumps

well I will take my downvote away, but I still do not agree with your answer, because I see it from a RAW standpoint (which is not enforced, but... well.)

well, let me quote the one rule that D&D5 gives just like any other edition... "ask your GM if you may do it".