So having a chat with my DM about the war domain's extra attack - war priest.

Can this be used after only doing an attack (melee or ranged) or does a spell count as an attack as long as its an attack roll e.g. guiding bolt?

So as long as you roll to hit a target be it melee, ranged, or a spell such as guiding bolt, chromatic orb, ray of frost etc, then that counts as an attack and thus I could then use the war priest bonus action to do a melee or ranged attack?

I always thought you would use the [Tooltip Not Found] action to cast a spell that required an attack roll. 

But that’s not the attack action.  The attack action is separate from the cast a spell action.  I understand that spells attack, but that’s not using the attack action which is specified as a melee or ranged attack. Your reference would consider the spell with an attack roll an attack, however that doesn’t specify it uses the attack action, it was an argument for a different case that isn’t quite the same (whether magic missile is considered an attack).

this isn’t questioning if something is an attack or not it’s questioning if a spell is considered an attack action ehich I don’t think it is.

Extra Attack requires the Attack action, not any action, such as Whirlwind Attack, that includes an attack (the proper nouns are key).

The Attack action is something you take on your turn (PH, 192). It, like many things in the game, includes attacking.

The Attack action involves an attack, but not every attack is delivered by the Attack action.

Extra Attack requires the Attack action, not any action, such as Whirlwind Attack, that includes an attack (the proper nouns are key).

Extra Attack relies on the Attack action. The Multiattack action is distinct from Attack and therefore doesn't work with Extra Attack.

This is also covered extensively in Sage Advice Compendium: 

Can a spell with an attack roll be used as the attack in the Attack action or as part of the Extra Attack feature?

The short answer is no.

As explained in the Player’s Handbook, you can take one action on your turn in combat, in addition to moving. You choose your action from the options available to everyone— options such as Attack, Cast a Spell, and Dash—or you choose from among the special actions you’ve gained from a class, a feat, or another source.

If you want to cast a spell on your turn, you take the Cast a Spell action. Doing so means you’re not taking the Attack action or any other action. It is true that a number of spells, such as fire bolt and ray of frost, involve making an attack, but you can’t make such an attack without first casting the spell that delivers it. In other words, just because something involves an attack doesn’t mean the Attack action is being used. By extension, the Extra Attack feature (given by several classes, including the fighter and paladin) doesn’t let you cast extra attack spells. That feature specifically relies on the Attack action, not the Cast a Spell action or any other action.

In summary, to make a spell attack, you have to first cast a spell or use a feature that creates the spell’s effect. A game feature, such as Extra Attack, that lets you make an attack doesn’t let you cast a spell unless it says it does.

Can you use a melee spell attack to make an opportunity attack?

You can’t if the spell attack is created by casting a spell. When a creature triggers an opportunity attack from you, you can use your reaction to make a melee attack against it. The opportunity attack doesn’t suddenly give you the ability to cast a spell, such as shocking grasp.

Each spell has a casting time. A game feature, such as an opportunity attack, doesn’t let you bypass that casting time, unless the feature says otherwise. The War Caster feat is an example of a feature that does let you bypass a 1-action casting time to cast a spell in place of making an opportunity attack.

A few monsters can make opportunity attacks with melee spell attacks. Here’s how: certain monsters—including the banshee, the lich, and the specter—have a melee spell attack that isn’t delivered by a spell. For example, the banshee’s Corrupting Touch action is a melee spell attack but no spell is cast to make it. The banshee can, therefore, make opportunity attacks with Corrupting Touch

 so I read this discussion and am stil confused on 1 thing. does war priest essntially have the same effect as the lv.5 feature for a few classes that allows them to attack twice, but it uses a bonus action???

@ Legendary_Punmaster:

Sorry it's taken a couple of days for someone to get around to answering your question.

The short answer is . . . kind of.

Yes, the War Priest feature allows a War cleric to make two weapon attacks on their turn, once with the Attack action, and once more with their bonus action.

The only problem with this is that in most cases, a cleric has many better things they can eventually do with their bonus action (or action) aside from making weapon attacks, unless it is a low-risk combat and you're feeling particularly stingy with your spell slots. Additionally, the ability can only be used a number of times equal to the cleric's Wisdom modifier per long rest (for most clerics, this number is between 3 and 5, and almost never more often than 5).

It is significantly less useful to the cleric than Extra Attack is for a martial class, because Strength/Dexterity (depending on the weapon being used) is a secondary (or even tertiary) ability for a cleric, Wisdom and (probably) Constitution being more important to them. That said, every other (non-multiclassed) cleric does not get more than one weapon attack, so that's something I guess. It would be much more useful if the War cleric's Divine Strike wasn't limited to just the one attack per round (can't even use it on attacks of opportunity, unlike a rogue's Sneak Attack [so unfair]).

The War cleric's Guided Strike/War God's Blessing feature is probably one of the subclass's best, giving yourself/an ally +10 to an attack roll (any attack roll, including those for spells!) is a really nice feature, and can be used multiple times per short rest at relatively low levels (starting at 6th).

Hope this info helps! I know I went a bit beyond the scope of your question, but I wanted to make sure you don't fall into a "trap" here.