`Instance:FindFirstChild()` and `Instance:FindFirstAncestor()` can't always be compared with `nil`

--!strict
local model = Instance.new("Model")
assert(model:FindFirstChild("does_not_exist") ~= nil) -- This warns.
assert(workspace:FindFirstChild("does_not_exist") ~= nil) -- This is fine.

TypeError: Types Instance and nil cannot be compared with ~= because they do not have the same metatable

[JohnnyMorganz]

Ah yeah, this is because FindFirstChild is currently typed as returning Instance instead of Instance?

I've been meaning to fix this, but not sure about potential impact. Especially in cases of find first child chains. What does studio type the return as?

[JohnnyMorganz]

Just an FYI about this currently, you can override the definitions used to support this, whilst you wait.
Its present at %appdata%\Code\User\globalStorage\johnnymorganz.luau-lsp\globalTypes.d.lua on Windows.
Note that the file will be overwritten whenever Roblox updates (which is weekly).

I'll look into the impact of changing it into an optional return

[JohnnyMorganz]

As an aside, #84 will add support to override the definitions file used completely, so that it won't get overwritten. Once environments are added, you can redefine the environment "roblox" with your own definitions file.