Merge type variable fixes and unit test (#1867)

Co-authored-by: Arkatos1 <43862960+Arkatos1@users.noreply.github.com>
Co-authored-by: Gandalf <jzo123@hotmail.com>

code/game/objects/items/devices/polycircuit.dm

@@ -5,6 +5,7 @@
 	inhand_icon_state = "rods"
 	w_class = WEIGHT_CLASS_TINY
 	max_amount = 8
+	merge_type = /obj/item/stack/circuit_stack
 	var/circuit_type = /obj/item/electronics/airlock
 	var/chosen_circuit = "airlock"
 

code/game/objects/items/stacks/ammonia_crystals.dm

@@ -7,3 +7,4 @@
 	resistance_flags = FLAMMABLE
 	max_amount = 50
 	grind_results = list(/datum/reagent/ammonia = 10)
+	merge_type = /obj/item/stack/ammonia_crystals

code/game/objects/items/stacks/bscrystal.dm

@@ -9,15 +9,18 @@
 	w_class = WEIGHT_CLASS_TINY
 	mats_per_unit = list(/datum/material/bluespace=MINERAL_MATERIAL_AMOUNT)
 	points = 50
-	var/blink_range = 8 // The teleport range when crushed/thrown at someone.
 	refined_type = /obj/item/stack/sheet/bluespace_crystal
 	grind_results = list(/datum/reagent/bluespace = 20)
 	scan_state = "rock_BScrystal"
+	merge_type = /obj/item/stack/ore/bluespace_crystal
+	/// The teleport range when crushed/thrown at someone.
+	var/blink_range = 8
 
 /obj/item/stack/ore/bluespace_crystal/refined
 	name = "refined bluespace crystal"
 	points = 0
 	refined_type = null
+	merge_type = /obj/item/stack/ore/bluespace_crystal/refined
 
 /obj/item/stack/ore/bluespace_crystal/Initialize()
 	. = ..()
@@ -56,6 +59,7 @@
 	points = 0 //nice try
 	refined_type = null
 	grind_results = list(/datum/reagent/bluespace = 10, /datum/reagent/silicon = 20)
+	merge_type = /obj/item/stack/ore/bluespace_crystal/artificial
 
 //Polycrystals, aka stacks
 /obj/item/stack/sheet/bluespace_crystal
@@ -71,6 +75,7 @@
 	novariants = TRUE
 	grind_results = list(/datum/reagent/bluespace = 20)
 	point_value = 30
+	merge_type = /obj/item/stack/sheet/bluespace_crystal
 	var/crystal_type = /obj/item/stack/ore/bluespace_crystal/refined
 
 /obj/item/stack/sheet/bluespace_crystal/attack_self(mob/user)// to prevent the construction menu from ever happening

code/game/objects/items/stacks/cash.dm

@@ -59,43 +59,52 @@
 	icon_state = "spacecash"
 	singular_name = "one credit bill"
 	value = 1
+	merge_type = /obj/item/stack/spacecash/c1
 
 /obj/item/stack/spacecash/c10
 	icon_state = "spacecash10"
 	singular_name = "ten credit bill"
 	value = 10
+	merge_type = /obj/item/stack/spacecash/c10
 
 /obj/item/stack/spacecash/c20
 	icon_state = "spacecash20"
 	singular_name = "twenty credit bill"
 	value = 20
+	merge_type = /obj/item/stack/spacecash/c20
 
 /obj/item/stack/spacecash/c50
 	icon_state = "spacecash50"
 	singular_name = "fifty credit bill"
 	value = 50
+	merge_type = /obj/item/stack/spacecash/c50
 
 /obj/item/stack/spacecash/c100
 	icon_state = "spacecash100"
 	singular_name = "one hundred credit bill"
 	value = 100
+	merge_type = /obj/item/stack/spacecash/c100
 
 /obj/item/stack/spacecash/c200
 	icon_state = "spacecash200"
 	singular_name = "two hundred credit bill"
 	value = 200
+	merge_type = /obj/item/stack/spacecash/c200
 
 /obj/item/stack/spacecash/c500
 	icon_state = "spacecash500"
 	singular_name = "five hundred credit bill"
 	value = 500
+	merge_type = /obj/item/stack/spacecash/c500
 
 /obj/item/stack/spacecash/c1000
 	icon_state = "spacecash1000"
 	singular_name = "one thousand credit bill"
 	value = 1000
+	merge_type = /obj/item/stack/spacecash/c1000
 
 /obj/item/stack/spacecash/c10000
 	icon_state = "spacecash10000"
 	singular_name = "ten thousand credit bill"
 	value = 10000
+	merge_type = /obj/item/stack/spacecash/c10000

code/game/objects/items/stacks/license_plates.dm

@@ -9,6 +9,7 @@
 /obj/item/stack/license_plates/empty
 	name = "empty license plate"
 	desc = "Instead of a license plate number, this could contain a quote like \"Live laugh love\"."
+	merge_type = /obj/item/stack/license_plates/empty
 
 /obj/item/stack/license_plates/empty/fifty
 	amount = 50
@@ -17,6 +18,7 @@
 	name = "license plate"
 	desc = "Prison labor paying off."
 	icon_state = "filled_plate_1_1"
+	merge_type = /obj/item/stack/license_plates/filled
 
 ///Override to allow for variations
 /obj/item/stack/license_plates/filled/update_icon_state()

code/game/objects/items/stacks/medical.dm

@@ -14,6 +14,7 @@
 	item_flags = NOBLUDGEON
 	cost = 250
 	source = /datum/robot_energy_storage/medical
+	merge_type = /obj/item/stack/medical
 	var/self_delay = 50
 	var/other_delay = 0
 	var/repeating = FALSE
@@ -90,6 +91,7 @@
 	self_delay = 40
 	other_delay = 20
 	grind_results = list(/datum/reagent/medicine/c2/libital = 10)
+	merge_type = /obj/item/stack/medical/bruise_pack
 
 /obj/item/stack/medical/bruise_pack/heal(mob/living/M, mob/user)
 	if(M.stat == DEAD)
@@ -129,6 +131,7 @@
 	absorption_rate = 0.25
 	absorption_capacity = 5
 	splint_factor = 0.35
+	merge_type = /obj/item/stack/medical/gauze
 
 // gauze is only relevant for wounds, which are handled in the wounds themselves
 /obj/item/stack/medical/gauze/try_heal(mob/living/M, mob/user, silent)
@@ -190,6 +193,7 @@
 	other_delay = 30
 	absorption_rate = 0.15
 	absorption_capacity = 4
+	merge_type = /obj/item/stack/medical/gauze/improvised
 
 /obj/item/stack/medical/suture
 	name = "suture"
@@ -205,6 +209,7 @@
 	heal_brute = 10
 	stop_bleeding = 0.6
 	grind_results = list(/datum/reagent/medicine/spaceacillin = 2)
+	merge_type = /obj/item/stack/medical/suture
 
 /obj/item/stack/medical/suture/emergency
 	name = "emergency suture"
@@ -212,6 +217,7 @@
 	heal_brute = 5
 	amount = 5
 	max_amount = 5
+	merge_type = /obj/item/stack/medical/suture/emergency
 
 /obj/item/stack/medical/suture/medicated
 	name = "medicated suture"
@@ -220,6 +226,7 @@
 	heal_brute = 15
 	stop_bleeding = 0.75
 	grind_results = list(/datum/reagent/medicine/polypyr = 1)
+	merge_type = /obj/item/stack/medical/suture/medicated
 
 /obj/item/stack/medical/suture/heal(mob/living/M, mob/user)
 	. = ..()
@@ -259,6 +266,7 @@
 	flesh_regeneration = 2.5
 	sanitization = 0.25
 	grind_results = list(/datum/reagent/medicine/c2/lenturi = 10)
+	merge_type = /obj/item/stack/medical/ointment
 
 /obj/item/stack/medical/ointment/heal(mob/living/M, mob/user)
 	if(M.stat == DEAD)
@@ -289,6 +297,7 @@
 
 	var/is_open = TRUE ///This var determines if the sterile packaging of the mesh has been opened.
 	grind_results = list(/datum/reagent/medicine/spaceacillin = 2)
+	merge_type = /obj/item/stack/medical/mesh
 
 /obj/item/stack/medical/mesh/Initialize()
 	. = ..()
@@ -349,6 +358,7 @@
 	sanitization = 1.25
 	flesh_regeneration = 3.5
 	grind_results = list(/datum/reagent/consumable/aloejuice = 1)
+	merge_type = /obj/item/stack/medical/mesh/advanced
 
 /obj/item/stack/medical/mesh/advanced/update_icon_state()
 	if(!is_open)
@@ -371,6 +381,7 @@
 	repeating = TRUE
 	var/heal = 3
 	grind_results = list(/datum/reagent/consumable/aloejuice = 1)
+	merge_type = /obj/item/stack/medical/aloe
 
 /obj/item/stack/medical/aloe/heal(mob/living/M, mob/user)
 	. = ..()
@@ -415,6 +426,7 @@
 	self_delay = 20
 	grind_results = list(/datum/reagent/medicine/c2/libital = 10)
 	novariants = TRUE
+	merge_type = /obj/item/stack/medical/bone_gel
 
 /obj/item/stack/medical/bone_gel/attack(mob/living/M, mob/user)
 	to_chat(user, "<span class='warning'>Bone gel can only be used on fractured limbs!</span>")
@@ -457,6 +469,7 @@
 	drop_sound = 'sound/misc/moist_impact.ogg'
 	mob_throw_hit_sound = 'sound/misc/moist_impact.ogg'
 	hitsound = 'sound/misc/moist_impact.ogg'
+	merge_type = /obj/item/stack/medical/poultice
 
 /obj/item/stack/medical/poultice/heal(mob/living/M, mob/user)
 	. = .. ()

code/game/objects/items/stacks/rods.dm

@@ -30,6 +30,7 @@ GLOBAL_LIST_INIT(rod_recipes, list ( \
 	matter_amount = 2
 	cost = 250
 	source = /datum/robot_energy_storage/metal
+	merge_type = /obj/item/stack/rods
 
 /obj/item/stack/rods/suicide_act(mob/living/carbon/user)
 	user.visible_message("<span class='suicide'>[user] begins to stuff \the [src] down [user.p_their()] throat! It looks like [user.p_theyre()] trying to commit suicide!</span>")//it looks like theyre ur mum
@@ -70,9 +71,6 @@ GLOBAL_LIST_INIT(rod_recipes, list ( \
 	else
 		return ..()
 
-/obj/item/stack/rods/cyborg
-	merge_type = /obj/item/stack/rods
-
 /obj/item/stack/rods/cyborg/ComponentInitialize()
 	. = ..()
 	AddElement(/datum/element/update_icon_blocker)
@@ -98,6 +96,7 @@ GLOBAL_LIST_INIT(rod_recipes, list ( \
 	mats_per_unit = list(/datum/material/iron=1000, /datum/material/plasma=500, /datum/material/titanium=2000)
 	max_amount = 30
 	resistance_flags = FIRE_PROOF | LAVA_PROOF
+	merge_type = /obj/item/stack/rods/lava
 
 /obj/item/stack/rods/lava/thirty
 	amount = 30

code/game/objects/items/stacks/sheets/hot_ice.dm

@@ -7,6 +7,7 @@
 	mats_per_unit = list(/datum/material/hot_ice=MINERAL_MATERIAL_AMOUNT)
 	grind_results = list(/datum/reagent/toxin/hot_ice = 25)
 	material_type = /datum/material/hot_ice
+	merge_type = /obj/item/stack/sheet/hot_ice
 
 /obj/item/stack/sheet/hot_ice/suicide_act(mob/living/carbon/user)
 	user.visible_message("<span class='suicide'>[user] begins licking \the [src]! It looks like [user.p_theyre()] trying to commit suicide!</span>")

code/game/objects/items/stacks/sheets/leather.dm

@@ -4,12 +4,14 @@
 	icon_state = "sheet-hide"
 	inhand_icon_state = "sheet-hide"
 	novariants = TRUE
+	merge_type = /obj/item/stack/sheet/animalhide
 
 /obj/item/stack/sheet/animalhide/human
 	name = "human skin"
 	desc = "The by-product of human farming."
 	singular_name = "human skin piece"
 	novariants = FALSE
+	merge_type = /obj/item/stack/sheet/animalhide/human
 
 GLOBAL_LIST_INIT(human_recipes, list( \
 	new/datum/stack_recipe("bloated human costume", /obj/item/clothing/suit/hooded/bloated_human, 5), \
@@ -24,6 +26,7 @@ GLOBAL_LIST_INIT(human_recipes, list( \
 	desc = "A piece of skin."
 	singular_name = "skin piece"
 	novariants = FALSE
+	merge_type = /obj/item/stack/sheet/animalhide/generic
 
 /obj/item/stack/sheet/animalhide/corgi
 	name = "corgi hide"
@@ -31,7 +34,7 @@ GLOBAL_LIST_INIT(human_recipes, list( \
 	singular_name = "corgi hide piece"
 	icon_state = "sheet-corgi"
 	inhand_icon_state = "sheet-corgi"
-
+	merge_type = /obj/item/stack/sheet/animalhide/corgi
 
 GLOBAL_LIST_INIT(gondola_recipes, list ( \
 	new/datum/stack_recipe("gondola mask", /obj/item/clothing/mask/gondola, 1), \
@@ -44,6 +47,7 @@ GLOBAL_LIST_INIT(gondola_recipes, list ( \
 	singular_name = "gondola hide piece"
 	icon_state = "sheet-gondola"
 	inhand_icon_state = "sheet-gondola"
+	merge_type = /obj/item/stack/sheet/animalhide/gondola
 
 /obj/item/stack/sheet/animalhide/gondola/get_main_recipes()
 	. = ..()
@@ -63,6 +67,7 @@ GLOBAL_LIST_INIT(corgi_recipes, list ( \
 	singular_name = "cat hide piece"
 	icon_state = "sheet-cat"
 	inhand_icon_state = "sheet-cat"
+	merge_type = /obj/item/stack/sheet/animalhide/cat
 
 /obj/item/stack/sheet/animalhide/monkey
 	name = "monkey hide"
@@ -70,6 +75,7 @@ GLOBAL_LIST_INIT(corgi_recipes, list ( \
 	singular_name = "monkey hide piece"
 	icon_state = "sheet-monkey"
 	inhand_icon_state = "sheet-monkey"
+	merge_type = /obj/item/stack/sheet/animalhide/monkey
 
 GLOBAL_LIST_INIT(monkey_recipes, list ( \
 	new/datum/stack_recipe("monkey mask", /obj/item/clothing/mask/gas/monkeymask, 1), \
@@ -86,6 +92,7 @@ GLOBAL_LIST_INIT(monkey_recipes, list ( \
 	singular_name = "lizard skin piece"
 	icon_state = "sheet-lizard"
 	inhand_icon_state = "sheet-lizard"
+	merge_type = /obj/item/stack/sheet/animalhide/lizard
 
 /obj/item/stack/sheet/animalhide/xeno
 	name = "alien hide"
@@ -93,6 +100,7 @@ GLOBAL_LIST_INIT(monkey_recipes, list ( \
 	singular_name = "alien hide piece"
 	icon_state = "sheet-xeno"
 	inhand_icon_state = "sheet-xeno"
+	merge_type = /obj/item/stack/sheet/animalhide/xeno
 
 GLOBAL_LIST_INIT(xeno_recipes, list ( \
 	new/datum/stack_recipe("alien helmet", /obj/item/clothing/head/xenos, 1), \
@@ -111,6 +119,7 @@ GLOBAL_LIST_INIT(xeno_recipes, list ( \
 	icon = 'icons/mob/alien.dmi'
 	icon_state = "chitin"
 	novariants = TRUE
+	merge_type = /obj/item/stack/sheet/xenochitin
 
 /obj/item/xenos_claw
 	name = "alien claw"
@@ -130,6 +139,7 @@ GLOBAL_LIST_INIT(xeno_recipes, list ( \
 	singular_name = "hairless hide piece"
 	icon_state = "sheet-hairlesshide"
 	inhand_icon_state = "sheet-hairlesshide"
+	merge_type = /obj/item/stack/sheet/hairlesshide
 
 /obj/item/stack/sheet/wethide
 	name = "wet hide"
@@ -137,8 +147,11 @@ GLOBAL_LIST_INIT(xeno_recipes, list ( \
 	singular_name = "wet hide piece"
 	icon_state = "sheet-wetleather"
 	inhand_icon_state = "sheet-wetleather"
-	var/wetness = 30 //Reduced when exposed to high temperautres
-	var/drying_threshold_temperature = 500 //Kelvin to start drying
+	merge_type = /obj/item/stack/sheet/wethide
+	/// Reduced when exposed to high temperatures
+	var/wetness = 30
+	/// Kelvin to start drying
+	var/drying_threshold_temperature = 500
 
 /obj/item/stack/sheet/wethide/Initialize(mapload, new_amount, merge)
 	. = ..()
@@ -153,6 +166,7 @@ GLOBAL_LIST_INIT(xeno_recipes, list ( \
 	singular_name = "leather piece"
 	icon_state = "sheet-leather"
 	inhand_icon_state = "sheet-leather"
+	merge_type = /obj/item/stack/sheet/leather
 
 GLOBAL_LIST_INIT(leather_recipes, list ( \
 	new/datum/stack_recipe("wallet", /obj/item/storage/wallet, 1), \
@@ -180,12 +194,13 @@ GLOBAL_LIST_INIT(leather_recipes, list ( \
 	singular_name = "watcher sinew"
 	icon_state = "sinew"
 	novariants = TRUE
+	merge_type = /obj/item/stack/sheet/sinew
 
 /obj/item/stack/sheet/sinew/wolf
 	name = "wolf sinew"
 	desc = "Long stringy filaments which came from the insides of a wolf."
 	singular_name = "wolf sinew"
-
+	merge_type = /obj/item/stack/sheet/sinew/wolf
 
 GLOBAL_LIST_INIT(sinew_recipes, list ( \
 	new/datum/stack_recipe("sinew restraints", /obj/item/restraints/handcuffs/cable/sinew, 1), \
@@ -195,9 +210,9 @@ GLOBAL_LIST_INIT(sinew_recipes, list ( \
 	. = ..()
 	. += GLOB.sinew_recipes
 
-		/*
+/*
  * Plates
- 		*/
+ */
 /obj/item/stack/sheet/animalhide/goliath_hide
 	name = "goliath hide plates"
 	desc = "Pieces of a goliath's rocky hide, these might be able to make your suit a bit more durable to attack from the local fauna."
@@ -209,12 +224,14 @@ GLOBAL_LIST_INIT(sinew_recipes, list ( \
 	item_flags = NOBLUDGEON
 	w_class = WEIGHT_CLASS_NORMAL
 	layer = MOB_LAYER
+	merge_type = /obj/item/stack/sheet/animalhide/goliath_hide
 
 /obj/item/stack/sheet/animalhide/goliath_hide/polar_bear_hide
 	name = "polar bear hides"
 	desc = "Pieces of a polar bear's fur, these might be able to make your suit a bit more durable to attack from the local fauna."
 	icon_state = "polar_bear_hide"
 	singular_name = "polar bear hide"
+	merge_type = /obj/item/stack/sheet/animalhide/goliath_hide/polar_bear_hide
 
 /obj/item/stack/sheet/animalhide/ashdrake
 	name = "ash drake hide"
@@ -227,7 +244,7 @@ GLOBAL_LIST_INIT(sinew_recipes, list ( \
 	item_flags = NOBLUDGEON
 	w_class = WEIGHT_CLASS_NORMAL
 	layer = MOB_LAYER
-
+	merge_type = /obj/item/stack/sheet/animalhide/ashdrake
 
 //Step one - dehairing.
 

code/game/objects/items/stacks/sheets/light.dm

@@ -12,6 +12,7 @@
 	flags_1 = CONDUCT_1
 	max_amount = 60
 	grind_results = list(/datum/reagent/silicon = 20, /datum/reagent/copper = 5)
+	merge_type = /obj/item/stack/light_w
 
 /obj/item/stack/light_w/attackby(obj/item/O, mob/user, params)
 	if(istype(O, /obj/item/stack/sheet/metal))

code/game/objects/items/stacks/sheets/sheet_types.dm

@@ -622,7 +622,7 @@ GLOBAL_LIST_INIT(bronze_recipes, list ( \
 	icon_state = "sheet-lessergem"
 	inhand_icon_state = "sheet-lessergem"
 	novariants = TRUE
-
+	merge_type = /obj/item/stack/sheet/lessergem
 
 /obj/item/stack/sheet/greatergem
 	name = "greater gems"
@@ -631,8 +631,9 @@ GLOBAL_LIST_INIT(bronze_recipes, list ( \
 	icon_state = "sheet-greatergem"
 	inhand_icon_state = "sheet-greatergem"
 	novariants = TRUE
+	merge_type = /obj/item/stack/sheet/greatergem
 
-	/*
+/*
  * Bones
  */
 /obj/item/stack/sheet/bone

code/game/objects/items/stacks/tape.dm

@@ -15,7 +15,7 @@
 	resistance_flags = FLAMMABLE
 	grind_results = list(/datum/reagent/cellulose = 5)
 	splint_factor = 0.8
-
+	merge_type = /obj/item/stack/sticky_tape
 	var/list/conferred_embed = EMBED_HARMLESS
 	var/overwrite_existing = FALSE
 
@@ -59,6 +59,7 @@
 	prefix = "super sticky"
 	conferred_embed = EMBED_HARMLESS_SUPERIOR
 	splint_factor = 0.6
+	merge_type = /obj/item/stack/sticky_tape/super
 
 /obj/item/stack/sticky_tape/pointy
 	name = "pointy tape"
@@ -67,6 +68,7 @@
 	icon_state = "tape_evil"
 	prefix = "pointy"
 	conferred_embed = EMBED_POINTY
+	merge_type = /obj/item/stack/sticky_tape/pointy
 
 /obj/item/stack/sticky_tape/pointy/super
 	name = "super pointy tape"
@@ -75,6 +77,7 @@
 	icon_state = "tape_spikes"
 	prefix = "super pointy"
 	conferred_embed = EMBED_POINTY_SUPERIOR
+	merge_type = /obj/item/stack/sticky_tape/pointy/super
 
 /obj/item/stack/sticky_tape/surgical
 	name = "surgical tape"
@@ -85,3 +88,4 @@
 	conferred_embed = list("embed_chance" = 30, "pain_mult" = 0, "jostle_pain_mult" = 0, "ignore_throwspeed_threshold" = TRUE)
 	splint_factor = 0.4
 	custom_price = PAYCHECK_MEDIUM
+	merge_type = /obj/item/stack/sticky_tape/surgical

code/game/objects/items/stacks/telecrystal.dm

@@ -8,6 +8,7 @@
 	w_class = WEIGHT_CLASS_TINY
 	max_amount = 50
 	item_flags = NOBLUDGEON
+	merge_type = /obj/item/stack/telecrystal
 
 /obj/item/stack/telecrystal/attack(mob/target, mob/user)
 	if(target == user) //You can't go around smacking people with crystals to find out if they have an uplink or not.

code/game/objects/items/stacks/tickets.dm

@@ -6,6 +6,7 @@
 	inhand_icon_state = "tickets"
 	w_class = WEIGHT_CLASS_TINY
 	max_amount = 30
+	merge_type = /obj/item/stack/arcadeticket
 
 /obj/item/stack/arcadeticket/Initialize(mapload, new_amount, merge = TRUE)
 	. = ..()

code/game/objects/items/stacks/tiles/light.dm

@@ -7,6 +7,7 @@
 	attack_verb_continuous = list("bashes", "batters", "bludgeons", "thrashes", "smashes")
 	attack_verb_simple = list("bash", "batter", "bludgeon", "thrash", "smash")
 	turf_type = /turf/open/floor/light
+	merge_type = /obj/item/stack/tile/light
 	var/state = 0
 
 /obj/item/stack/tile/light/attackby(obj/item/O, mob/user, params)

code/game/objects/items/stacks/tiles/tile_mineral.dm

@@ -7,6 +7,7 @@
 	turf_type = /turf/open/floor/mineral/plasma
 	mineralType = "plasma"
 	mats_per_unit = list(/datum/material/plasma=500)
+	merge_type = /obj/item/stack/tile/mineral/plasma
 
 /obj/item/stack/tile/mineral/uranium
 	name = "uranium tile"
@@ -17,6 +18,7 @@
 	turf_type = /turf/open/floor/mineral/uranium
 	mineralType = "uranium"
 	mats_per_unit = list(/datum/material/uranium=500)
+	merge_type = /obj/item/stack/tile/mineral/uranium
 
 /obj/item/stack/tile/mineral/gold
 	name = "gold tile"
@@ -27,6 +29,7 @@
 	turf_type = /turf/open/floor/mineral/gold
 	mineralType = "gold"
 	mats_per_unit = list(/datum/material/gold=500)
+	merge_type = /obj/item/stack/tile/mineral/gold
 
 /obj/item/stack/tile/mineral/silver
 	name = "silver tile"
@@ -37,6 +40,7 @@
 	turf_type = /turf/open/floor/mineral/silver
 	mineralType = "silver"
 	mats_per_unit = list(/datum/material/silver=500)
+	merge_type = /obj/item/stack/tile/mineral/silver
 
 /obj/item/stack/tile/mineral/diamond
 	name = "diamond tile"
@@ -47,6 +51,7 @@
 	turf_type = /turf/open/floor/mineral/diamond
 	mineralType = "diamond"
 	mats_per_unit = list(/datum/material/diamond=500)
+	merge_type = /obj/item/stack/tile/mineral/diamond
 
 /obj/item/stack/tile/mineral/bananium
 	name = "bananium tile"
@@ -57,6 +62,7 @@
 	turf_type = /turf/open/floor/mineral/bananium
 	mineralType = "bananium"
 	mats_per_unit = list(/datum/material/bananium=500)
+	merge_type = /obj/item/stack/tile/mineral/bananium
 
 /obj/item/stack/tile/mineral/abductor
 	name = "alien floor tile"
@@ -68,6 +74,7 @@
 	mats_per_unit = list(/datum/material/alloy/alien=MINERAL_MATERIAL_AMOUNT*0.25)
 	turf_type = /turf/open/floor/mineral/abductor
 	mineralType = "abductor"
+	merge_type = /obj/item/stack/tile/mineral/abductor
 
 /obj/item/stack/tile/mineral/titanium
 	name = "titanium tile"
@@ -78,6 +85,7 @@
 	turf_type = /turf/open/floor/mineral/titanium
 	mineralType = "titanium"
 	mats_per_unit = list(/datum/material/titanium=500)
+	merge_type = /obj/item/stack/tile/mineral/titanium
 	tile_reskin_types = list(
 		/obj/item/stack/tile/mineral/titanium,
 		/obj/item/stack/tile/mineral/titanium/yellow,
@@ -97,6 +105,7 @@
 	desc = "Sleek yellow titanium tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/yellow
 	icon_state = "tile_titanium_yellow"
+	merge_type = /obj/item/stack/tile/mineral/titanium/yellow
 
 /obj/item/stack/tile/mineral/titanium/blue
 	name = "blue titanium tile"
@@ -104,6 +113,7 @@
 	desc = "Sleek blue titanium tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/blue
 	icon_state = "tile_titanium_blue"
+	merge_type = /obj/item/stack/tile/mineral/titanium/blue
 
 /obj/item/stack/tile/mineral/titanium/white
 	name = "white titanium tile"
@@ -111,6 +121,7 @@
 	desc = "Sleek white titanium tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/white
 	icon_state = "tile_titanium_white"
+	merge_type = /obj/item/stack/tile/mineral/titanium/white
 
 /obj/item/stack/tile/mineral/titanium/purple
 	name = "purple titanium tile"
@@ -118,6 +129,7 @@
 	desc = "Sleek purple titanium tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/purple
 	icon_state = "tile_titanium_purple"
+	merge_type = /obj/item/stack/tile/mineral/titanium/purple
 
 /obj/item/stack/tile/mineral/titanium/tiled
 	name = "tiled titanium tile"
@@ -125,6 +137,7 @@
 	desc = "Titanium floor tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/tiled
 	icon_state = "tile_titanium_tiled"
+	merge_type = /obj/item/stack/tile/mineral/titanium/tiled
 
 /obj/item/stack/tile/mineral/titanium/tiled/yellow
 	name = "yellow titanium tile"
@@ -132,6 +145,7 @@
 	desc = "Yellow titanium floor tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/tiled/yellow
 	icon_state = "tile_titanium_tiled_yellow"
+	merge_type = /obj/item/stack/tile/mineral/titanium/tiled/yellow
 
 /obj/item/stack/tile/mineral/titanium/tiled/blue
 	name = "blue titanium tile"
@@ -139,6 +153,7 @@
 	desc = "Blue titanium floor tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/tiled/blue
 	icon_state = "tile_titanium_tiled_blue"
+	merge_type = /obj/item/stack/tile/mineral/titanium/tiled/blue
 
 /obj/item/stack/tile/mineral/titanium/tiled/white
 	name = "white titanium tile"
@@ -146,6 +161,7 @@
 	desc = "White titanium floor tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/tiled/white
 	icon_state = "tile_titanium_tiled_white"
+	merge_type = /obj/item/stack/tile/mineral/titanium/tiled/white
 
 /obj/item/stack/tile/mineral/titanium/tiled/purple
 	name = "purple titanium tile"
@@ -153,6 +169,7 @@
 	desc = "Purple titanium floor tiles. Use while in your hand to change what type of titanium tiles you want."
 	turf_type = /turf/open/floor/mineral/titanium/tiled/purple
 	icon_state = "tile_titanium_tiled_purple"
+	merge_type = /obj/item/stack/tile/mineral/titanium/tiled/purple
 
 /obj/item/stack/tile/mineral/plastitanium
 	name = "plastitanium tile"
@@ -164,6 +181,7 @@
 	mineralType = "plastitanium"
 	mats_per_unit = list(/datum/material/alloy/plastitanium=MINERAL_MATERIAL_AMOUNT*0.25)
 	material_flags = MATERIAL_NO_EFFECTS
+	merge_type = /obj/item/stack/tile/mineral/plastitanium
 
 /obj/item/stack/tile/mineral/snow
 	name = "snow tile"
@@ -173,3 +191,4 @@
 	inhand_icon_state = "tile-silver"
 	turf_type = /turf/open/floor/grass/snow/safe
 	mineralType = "snow"
+	merge_type = /obj/item/stack/tile/mineral/snow

code/game/objects/items/stacks/tiles/tile_types.dm

@@ -103,6 +103,7 @@
 	inhand_icon_state = "tile-grass"
 	turf_type = /turf/open/floor/grass
 	resistance_flags = FLAMMABLE
+	merge_type = /obj/item/stack/tile/grass
 
 //Fairygrass
 /obj/item/stack/tile/fairygrass
@@ -113,6 +114,7 @@
 	inhand_icon_state = "tile-fairygrass"
 	turf_type = /turf/open/floor/grass/fairy
 	resistance_flags = FLAMMABLE
+	merge_type = /obj/item/stack/tile/fairygrass
 
 //Wood
 /obj/item/stack/tile/wood
@@ -123,6 +125,7 @@
 	inhand_icon_state = "tile-wood"
 	turf_type = /turf/open/floor/wood
 	resistance_flags = FLAMMABLE
+	merge_type = /obj/item/stack/tile/wood
 
 //Basalt
 /obj/item/stack/tile/basalt
@@ -132,6 +135,7 @@
 	icon_state = "tile_basalt"
 	inhand_icon_state = "tile-basalt"
 	turf_type = /turf/open/floor/grass/fakebasalt
+	merge_type = /obj/item/stack/tile/basalt
 
 //Carpets
 /obj/item/stack/tile/carpet
@@ -143,6 +147,7 @@
 	turf_type = /turf/open/floor/carpet
 	resistance_flags = FLAMMABLE
 	tableVariant = /obj/structure/table/wood/fancy
+	merge_type = /obj/item/stack/tile/carpet
 
 /obj/item/stack/tile/carpet/black
 	name = "black carpet"
@@ -150,6 +155,7 @@
 	inhand_icon_state = "tile-carpet-black"
 	turf_type = /turf/open/floor/carpet/black
 	tableVariant = /obj/structure/table/wood/fancy/black
+	merge_type = /obj/item/stack/tile/carpet/black
 
 /obj/item/stack/tile/carpet/blue
 	name = "blue carpet"
@@ -157,6 +163,7 @@
 	inhand_icon_state = "tile-carpet-blue"
 	turf_type = /turf/open/floor/carpet/blue
 	tableVariant = /obj/structure/table/wood/fancy/blue
+	merge_type = /obj/item/stack/tile/carpet/blue
 
 /obj/item/stack/tile/carpet/cyan
 	name = "cyan carpet"
@@ -164,6 +171,7 @@
 	inhand_icon_state = "tile-carpet-cyan"
 	turf_type = /turf/open/floor/carpet/cyan
 	tableVariant = /obj/structure/table/wood/fancy/cyan
+	merge_type = /obj/item/stack/tile/carpet/cyan
 
 /obj/item/stack/tile/carpet/green
 	name = "green carpet"
@@ -171,6 +179,7 @@
 	inhand_icon_state = "tile-carpet-green"
 	turf_type = /turf/open/floor/carpet/green
 	tableVariant = /obj/structure/table/wood/fancy/green
+	merge_type = /obj/item/stack/tile/carpet/green
 
 /obj/item/stack/tile/carpet/orange
 	name = "orange carpet"
@@ -178,6 +187,7 @@
 	inhand_icon_state = "tile-carpet-orange"
 	turf_type = /turf/open/floor/carpet/orange
 	tableVariant = /obj/structure/table/wood/fancy/orange
+	merge_type = /obj/item/stack/tile/carpet/orange
 
 /obj/item/stack/tile/carpet/purple
 	name = "purple carpet"
@@ -185,6 +195,7 @@
 	inhand_icon_state = "tile-carpet-purple"
 	turf_type = /turf/open/floor/carpet/purple
 	tableVariant = /obj/structure/table/wood/fancy/purple
+	merge_type = /obj/item/stack/tile/carpet/purple
 
 /obj/item/stack/tile/carpet/red
 	name = "red carpet"
@@ -192,6 +203,7 @@
 	inhand_icon_state = "tile-carpet-red"
 	turf_type = /turf/open/floor/carpet/red
 	tableVariant = /obj/structure/table/wood/fancy/red
+	merge_type = /obj/item/stack/tile/carpet/red
 
 /obj/item/stack/tile/carpet/royalblack
 	name = "royal black carpet"
@@ -199,6 +211,7 @@
 	inhand_icon_state = "tile-carpet-royalblack"
 	turf_type = /turf/open/floor/carpet/royalblack
 	tableVariant = /obj/structure/table/wood/fancy/royalblack
+	merge_type = /obj/item/stack/tile/carpet/royalblack
 
 /obj/item/stack/tile/carpet/royalblue
 	name = "royal blue carpet"
@@ -206,24 +219,28 @@
 	inhand_icon_state = "tile-carpet-royalblue"
 	turf_type = /turf/open/floor/carpet/royalblue
 	tableVariant = /obj/structure/table/wood/fancy/royalblue
+	merge_type = /obj/item/stack/tile/carpet/royalblue
 
 /obj/item/stack/tile/carpet/executive
 	name = "executive carpet"
 	icon_state = "tile_carpet_executive"
 	inhand_icon_state = "tile-carpet-royalblue"
 	turf_type = /turf/open/floor/carpet/executive
+	merge_type = /obj/item/stack/tile/carpet/executive
 
 /obj/item/stack/tile/carpet/stellar
 	name = "stellar carpet"
 	icon_state = "tile_carpet_stellar"
 	inhand_icon_state = "tile-carpet-royalblue"
 	turf_type = /turf/open/floor/carpet/stellar
+	merge_type = /obj/item/stack/tile/carpet/stellar
 
 /obj/item/stack/tile/carpet/donk
 	name = "donk co promotional carpet"
 	icon_state = "tile_carpet_donk"
 	inhand_icon_state = "tile-carpet-orange"
 	turf_type = /turf/open/floor/carpet/donk
+	merge_type = /obj/item/stack/tile/carpet/donk
 
 /obj/item/stack/tile/carpet/fifty
 	amount = 50
@@ -311,6 +328,7 @@
 	icon_state = "tile_bcircuit"
 	inhand_icon_state = "tile-bcircuit"
 	turf_type = /turf/open/floor/circuit
+	merge_type = /obj/item/stack/tile/circuit
 
 /obj/item/stack/tile/circuit/green
 	name = "green circuit tile"
@@ -319,9 +337,11 @@
 	icon_state = "tile_gcircuit"
 	inhand_icon_state = "tile-gcircuit"
 	turf_type = /turf/open/floor/circuit/green
+	merge_type = /obj/item/stack/tile/circuit/green
 
 /obj/item/stack/tile/circuit/green/anim
 	turf_type = /turf/open/floor/circuit/green/anim
+	merge_type = /obj/item/stack/tile/circuit/green/anim
 
 /obj/item/stack/tile/circuit/red
 	name = "red circuit tile"
@@ -330,9 +350,11 @@
 	icon_state = "tile_rcircuit"
 	inhand_icon_state = "tile-rcircuit"
 	turf_type = /turf/open/floor/circuit/red
+	merge_type = /obj/item/stack/tile/circuit/red
 
 /obj/item/stack/tile/circuit/red/anim
 	turf_type = /turf/open/floor/circuit/red/anim
+	merge_type = /obj/item/stack/tile/circuit/red/anim
 
 //Pod floor
 /obj/item/stack/tile/pod
@@ -342,6 +364,7 @@
 	icon_state = "tile_pod"
 	inhand_icon_state = "tile-pod"
 	turf_type = /turf/open/floor/pod
+	merge_type = /obj/item/stack/tile/pod
 
 /obj/item/stack/tile/pod/light
 	name = "light pod floor tile"
@@ -349,6 +372,7 @@
 	desc = "A lightly colored grooved floor tile."
 	icon_state = "tile_podlight"
 	turf_type = /turf/open/floor/pod/light
+	merge_type = /obj/item/stack/tile/pod/light
 
 /obj/item/stack/tile/pod/dark
 	name = "dark pod floor tile"
@@ -356,6 +380,7 @@
 	desc = "A darkly colored grooved floor tile."
 	icon_state = "tile_poddark"
 	turf_type = /turf/open/floor/pod/dark
+	merge_type = /obj/item/stack/tile/pod/dark
 
 //Plasteel (normal)
 /obj/item/stack/tile/plasteel
@@ -375,6 +400,7 @@
 	matter_amount = 1
 	cost = 125
 	source = /datum/robot_energy_storage/metal
+	merge_type = /obj/item/stack/tile/plasteel
 
 /obj/item/stack/tile/plastic
 	name = "plastic tile"
@@ -383,6 +409,7 @@
 	icon_state = "tile_plastic"
 	mats_per_unit = list(/datum/material/plastic=500)
 	turf_type = /turf/open/floor/plastic
+	merge_type = /obj/item/stack/tile/plastic
 
 /obj/item/stack/tile/material
 	name = "floor tile"
@@ -392,6 +419,7 @@
 	icon_state = "material_tile"
 	turf_type = /turf/open/floor/material
 	material_flags = MATERIAL_ADD_PREFIX | MATERIAL_COLOR | MATERIAL_AFFECT_STATISTICS
+	merge_type = /obj/item/stack/tile/material
 
 /obj/item/stack/tile/eighties
 	name = "retro tile"
@@ -399,6 +427,7 @@
 	desc = "A stack of floor tiles that remind you of an age of funk."
 	icon_state = "tile_eighties"
 	turf_type = /turf/open/floor/eighties
+	merge_type = /obj/item/stack/tile/eighties
 
 /obj/item/stack/tile/eighties/loaded
 	amount = 15

code/game/objects/items/stacks/wrap.dm

@@ -13,6 +13,7 @@
 	amount = 25
 	max_amount = 25
 	resistance_flags = FLAMMABLE
+	merge_type = /obj/item/stack/wrapping_paper
 
 /obj/item/stack/wrapping_paper/use(used, transfer)
 	var/turf/T = get_turf(src)
@@ -23,6 +24,7 @@
 /obj/item/stack/wrapping_paper/small
 	desc = "Wrap packages with this festive paper to make gifts. This roll looks a bit skimpy."
 	amount = 10
+	merge_type = /obj/item/stack/wrapping_paper/small
 
 /*
  * Package Wrap
@@ -39,6 +41,7 @@
 	max_amount = 25
 	resistance_flags = FLAMMABLE
 	grind_results = list(/datum/reagent/cellulose = 5)
+	merge_type = /obj/item/stack/package_wrap
 
 /obj/item/stack/package_wrap/suicide_act(mob/living/user)
 	user.visible_message("<span class='suicide'>[user] begins wrapping [user.p_them()]self in \the [src]! It looks like [user.p_theyre()] trying to commit suicide!</span>")
@@ -128,6 +131,7 @@
 	desc = "You can use this to wrap items in. This roll looks a bit skimpy."
 	w_class = WEIGHT_CLASS_SMALL
 	amount = 5
+	merge_type = /obj/item/stack/package_wrap/small
 
 /obj/item/c_tube
 	name = "cardboard tube"

code/modules/events/crystal_event.dm

@@ -477,6 +477,7 @@ This section is for the crystal stabilizer item and the crystal from the closed
 	singular_name = "otherworld crystal"
 	icon = 'icons/obj/stack_objects.dmi'
 	material_type = /datum/material/otherworld_crystal
+	merge_type = /obj/item/stack/sheet/otherworld_crystal
 
 /*
 This section is for the signaler part of the crystal portals

code/modules/mining/ores_coins.dm

@@ -78,6 +78,7 @@
 	mine_experience = 6
 	scan_state = "rock_Uranium"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/uranium
 
 /obj/item/stack/ore/iron
 	name = "iron ore"
@@ -90,6 +91,7 @@
 	mine_experience = 1
 	scan_state = "rock_Iron"
 	spreadChance = 20
+	merge_type = /obj/item/stack/ore/iron
 
 /obj/item/stack/ore/glass
 	name = "sand pile"
@@ -101,6 +103,7 @@
 	refined_type = /obj/item/stack/sheet/glass
 	w_class = WEIGHT_CLASS_TINY
 	mine_experience = 0 //its sand
+	merge_type = /obj/item/stack/ore/glass
 
 GLOBAL_LIST_INIT(sand_recipes, list(\
 		new /datum/stack_recipe("sandstone", /obj/item/stack/sheet/mineral/sandstone, 1, 1, 50),\
@@ -135,6 +138,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	inhand_icon_state = "volcanic_sand"
 	singular_name = "volcanic ash pile"
 	mine_experience = 0
+	merge_type = /obj/item/stack/ore/glass/basalt
 
 /obj/item/stack/ore/plasma
 	name = "plasma ore"
@@ -147,12 +151,12 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	mine_experience = 5
 	scan_state = "rock_Plasma"
 	spreadChance = 8
+	merge_type = /obj/item/stack/ore/plasma
 
 /obj/item/stack/ore/plasma/welder_act(mob/living/user, obj/item/I)
 	to_chat(user, "<span class='warning'>You can't hit a high enough temperature to smelt [src] properly!</span>")
 	return TRUE
 
-
 /obj/item/stack/ore/silver
 	name = "silver ore"
 	icon_state = "Silver ore"
@@ -164,6 +168,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/silver
 	scan_state = "rock_Silver"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/silver
 
 /obj/item/stack/ore/gold
 	name = "gold ore"
@@ -176,6 +181,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/gold
 	scan_state = "rock_Gold"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/gold
 
 /obj/item/stack/ore/diamond
 	name = "diamond ore"
@@ -187,6 +193,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/diamond
 	mine_experience = 10
 	scan_state = "rock_Diamond"
+	merge_type = /obj/item/stack/ore/diamond
 
 /obj/item/stack/ore/bananium
 	name = "bananium ore"
@@ -198,6 +205,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/bananium
 	mine_experience = 15
 	scan_state = "rock_Bananium"
+	merge_type = /obj/item/stack/ore/bananium
 
 /obj/item/stack/ore/titanium
 	name = "titanium ore"
@@ -210,6 +218,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	mine_experience = 3
 	scan_state = "rock_Titanium"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/titanium
 
 /obj/item/stack/ore/slag
 	name = "slag"
@@ -217,6 +226,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	icon_state = "slag"
 	inhand_icon_state = "slag"
 	singular_name = "slag chunk"
+	merge_type = /obj/item/stack/ore/slag
 
 /obj/item/gibtonite
 	name = "gibtonite ore"

code/modules/recycling/conveyor2.dm

@@ -421,6 +421,7 @@ GLOBAL_LIST_EMPTY(conveyors_by_id)
 	max_amount = 30
 	singular_name = "conveyor belt"
 	w_class = WEIGHT_CLASS_BULKY
+	merge_type = /obj/item/stack/conveyor
 	///id for linking
 	var/id = ""
 

code/modules/research/xenobiology/xenobiology.dm

@@ -1044,7 +1044,7 @@
 	flags_1 = CONDUCT_1
 	max_amount = 60
 	turf_type = /turf/open/floor/bluespace
-
+	merge_type = /obj/item/stack/tile/bluespace
 
 /obj/item/stack/tile/sepia
 	name = "sepia floor tile"
@@ -1061,7 +1061,7 @@
 	flags_1 = CONDUCT_1
 	max_amount = 60
 	turf_type = /turf/open/floor/sepia
-
+	merge_type = /obj/item/stack/tile/sepia
 
 /obj/item/areaeditor/blueprints/slime
 	name = "cerulean prints"

code/modules/unit_tests/_unit_tests.dm

@@ -45,6 +45,7 @@
 #include "keybinding_init.dm"
 #include "machine_disassembly.dm"
 #include "medical_wounds.dm"
+#include "merge_type.dm"
 #include "metabolizing.dm"
 #include "outfit_sanity.dm"
 #include "pills.dm"

code/modules/unit_tests/merge_type.dm

@@ -0,0 +1,15 @@
+/datum/unit_test/merge_type/Run()
+	var/list/blacklist = list(/obj/item/stack/sheet,
+							  /obj/item/stack/sheet/mineral,
+							  /obj/item/stack/ore,
+							  /obj/item/stack/spacecash,
+							  /obj/item/stack/license_plates,
+							  /obj/item/stack/tile/mineral,
+							  /obj/item/stack/tile)
+
+	var/list/paths = subtypesof(/obj/item/stack) - blacklist
+
+	for(var/stackpath in paths)
+		var/obj/item/stack/stack = stackpath
+		if(!initial(stack.merge_type))
+			Fail("([stack]) lacks set merge_type variable!")