custom wieldhand: toolcaps not used if not wielding empty slot

[sofar]

Expectation:

If I make a custom wieldhand and equip it, when I wield a node without toolcaps, the equipped "hand" slot itemstack toolcaps should be used.

So let's say I make a "chainsaw hand" that can saw through wood fast, and equip it. Then I should expect it to saw fast even if I don't have an empty wieldhand.

Reality:

This works for the builtin wieldhand, but not custom wieldhands.

If you set a custom wieldhand, the toolcaps are not used if you wield a node (e.g. dirt node)

steps to reproduce:

1. default wieldhand - remove all tool caps so the default wieldhand can't dig a single node
2. make custom wieldhand itemstack with tool caps that allow digging nodes
3. use inventory:set_stack("hand", 1, "custom_hand")
4. observe player wields new hand if you have an empty itemstack selected in the inventory
5. dig a node -> you can dig the node with the custom hand
6. now you receive a node in the empty inventory slot, and your wielditem shows the node
7. you can no longer dig nodes as long as you wield the node

[sofar]

Reproduction code:

minetest.register_item("hand:super", {
	type = "none",
	description = "super hand",
	wield_image = "wieldhand.png",
	inventory_image = "wieldhand.png",
	wield_scale = {x = 1, y = 1, z = 2},
	range = 6,
	tool_capabilities = {
		full_punch_interval = 0.1,
		max_drop_level = 0,
		groupcaps = {
			choppy = {times={[1] = 0.2, [2] = 0.2, [3] = 0.2}, uses = 0, maxlevel = 1},
			crumbly = {times={[1] = 0.2, [2] = 0.2, [3] = 0.2}, uses = 0, maxlevel = 1},
			cracky = {times={[1] = 0.2, [2] = 0.2, [3] = 0.2}, uses = 0, maxlevel = 1},
		},
		damage_groups = {},
	},
	groups = {not_in_creative_inventory = 1},
})

minetest.register_on_joinplayer(function(player)
	player:get_inventory():set_stack("hand", 1, "hand:super")
end)

Throw this in a minimal game and call it hand/init.lua and enable.

If you have an empty slot selected, you can dig any node. If there is a node in the slot, you can't dig e.g. stone anymore.

[sofar]

Root cause:

https://github.com/minetest/minetest/blob/f2f9a923515386d787a245fac52f78e815b3a839/src/inventory.h#L116

The hand slot is never inspected, it always tries the default hand item only.

[sofar]

I'm lost in const * c++, I really don't know C++ well enough to fix this. Help?

[sofar]

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