Uppercases slot defines.

code/modules/clothing/glasses/_glasses.dm

@@ -4,7 +4,7 @@
 	icon = 'icons/obj/clothing/glasses.dmi'
 	w_class = WEIGHT_CLASS_SMALL
 	flags_cover = GLASSESCOVERSEYES
-	slot_flags = SLOT_EYES
+	slot_flags = ITEM_SLOT_EYES
 	strip_delay = 20
 	equip_delay_other = 25
 	resistance_flags = NONE
@@ -100,7 +100,7 @@
 	armor = list("melee" = 0, "bullet" = 0, "laser" = 0, "energy" = 0, "bomb" = 0, "bio" = 0, "rad" = 0, "fire" = 80, "acid" = 100)
 
 /obj/item/clothing/glasses/science/item_action_slot_check(slot)
-	if(slot == slot_glasses)
+	if(slot == SLOT_GLASSES)
 		return 1
 
 /obj/item/clothing/glasses/night
@@ -263,7 +263,7 @@
 
 /obj/item/clothing/glasses/sunglasses/blindfold/equipped(mob/living/carbon/human/user, slot)
 	. = ..()
-	if(slot == slot_glasses)
+	if(slot == SLOT_GLASSES)
 		user.become_blind("blindfold_[REF(src)]")
 
 /obj/item/clothing/glasses/sunglasses/blindfold/dropped(mob/living/carbon/human/user)

code/modules/clothing/glasses/engine_goggles.dm

@@ -123,7 +123,7 @@
 	item_state = icon_state
 	if(isliving(loc))
 		var/mob/living/user = loc
-		if(user.get_item_by_slot(slot_glasses) == src)
+		if(user.get_item_by_slot(SLOT_GLASSES) == src)
 			user.update_inv_glasses()
 		else
 			user.update_inv_hands()

code/modules/clothing/glasses/hud.dm

@@ -6,7 +6,7 @@
 
 /obj/item/clothing/glasses/hud/equipped(mob/living/carbon/human/user, slot)
 	..()
-	if(hud_type && slot == slot_glasses)
+	if(hud_type && slot == SLOT_GLASSES)
 		var/datum/atom_hud/H = GLOB.huds[hud_type]
 		H.add_hud_to(user)