Calling getmetatable on a table union causes type checker to crash

[goldenstein64 (Shoop)]

Calling getmetatable on this unioned table type from a function:

type Table = { 
	property: any,
	[any]: any?
} | {}

causes Studio to crash only when:

• the new Lua script analysis beta feature is on,
• the Table type is referenced as an argument to a function,
• getmetatable is called on that argument,
• the two table types are unioned in that specific order,
• the property token is 3 characters or larger,
• and the function token is 3 characters or larger

The crash may not occur immediately when pasted. Undoing and repasting the code will eventually cause studio to crash, however. The minimum amount of repasting I had to do was only five times, and that only happened once in my own testing.

I just discovered this bug today.

Repro

1. Create a new baseplate place,
2. Paste this code into any Luau source container:

type T = {
	___: any,
	[any]: any
} | {}

local function ___(t: T)
	getmetatable(t)
end

3. Studio should crash immediately, and if not, undo the operation and repeat step 2.

Specs

OS: Windows 10
CPU: AMD Ryzen 7 1700 Eight-Core Processor
Memory: 16.0 GB
GPU: Radeon RX 550

Crash Dump

https://devforum.roblox.com/t/file-for-calling-getmetatable-on-a-table-union-causes-type-checker-to-crash/582773?u=goldenstein64

[Apakovtac]

This was a good report! I have a fix for this going in next week.

FYI, it does not require 3 characters or more for those name. It also doesn’t specifically require getmetatable either. However, you are correct in that it does require that there is:

• {x: number, [any]: any} | {} - in this order.
• you have to have a function taking a property that doesn’t exist in either member of the union.

Which means this is also another repro:

type T = {x: number, [any]: any} | {}
local t: T

function f(a)
  a.y = 1
end

f(t)