toStrictEqual incorrectly rejects equal primitives with locked metatables

[christopher-buss]

toStrictEqual fails for any comparison involving string values. typeEquality calls getmetatable() on primitives, and Roblox Luau strings have a locked metatable — getmetatable("hello") returns "The metatable is locked" instead of nil. This causes typeEquality to short-circuit with false before Object.is is consulted, so two identical strings are treated as unequal.

These fail in jest-roblox but pass in Jest:

-- Fails: "serializes to the same string"
expect({ value = "hello" }).toStrictEqual({ value = "hello" })

-- Works
expect({ value = "hello" }).toEqual({ value = "hello" })

Any toStrictEqual comparison that recurses into string (or other locked-metatable primitive) leaf values will hit this.

Cause

typeEquality in expect/src/utils.lua runs as a custom tester on every recursive pair of values, including leaf primitives:

local function typeEquality(a, b)
	if a == nil or b == nil then
		return nil
	end

	if typeof(a) ~= typeof(b) then
		return false
	end

	-- Path A: both no metatable -> pass through
	if not getmetatable(a) and not getmetatable(b) then
		return nil
	end

	-- Path B: both have metatables with matching __index -> pass through
	if
		getmetatable(a)
		and getmetatable(b)
		and getmetatable(a).__index
		and getmetatable(b).__index
		and getmetatable(a).__index == getmetatable(b).__index
	then
		return nil
	end

	-- Path C: FAIL
	return false
end

When a and b are both strings (e.g. "hello"):

1. Path A is skipped: getmetatable("hello") returns "The metatable is locked" (truthy), so not getmetatable(a) is false.
2. Path B fails: getmetatable("hello").__index indexes a string with "__index", which resolves through the string library to nil. Condition fails.
3. Path C is reached: return false. Two identical strings declared unequal.

Custom testers run before Object.is in eq(), so the correct comparison never happens:

for _, value in ipairs(customTesters) do
	local customTesterResult = value(a, b)
	if customTesterResult ~= nil then
		return customTesterResult -- returns false for strings
	end
end

-- never reached
if Object.is(a, b) then
	return true
end

Object.is("hello", "hello") would return true, but it's never called.

Proposed fix

Skip non-table metatables in typeEquality:

local function typeEquality(a, b)
	if a == nil or b == nil then
		return nil
	end
	if typeof(a) ~= typeof(b) then
		return false
	end

	local metaA = getmetatable(a)
	local metaB = getmetatable(b)

	-- Skip non-table metatables (e.g. "The metatable is locked" on strings)
	if typeof(metaA) ~= "table" and typeof(metaB) ~= "table" then
		return nil
	end

	if not metaA and not metaB then
		return nil
	end

	if
		metaA
		and metaB
		and typeof(metaA) == "table"
		and typeof(metaB) == "table"
		and metaA.__index
		and metaB.__index
		and metaA.__index == metaB.__index
	then
		return nil
	end

	return false
end

Or just bail early on non-tables:

if typeof(a) ~= "table" then
	return nil
end

Repro

local JestGlobals = require(Packages.Dev.JestGlobals)
local describe = JestGlobals.describe
local expect = JestGlobals.expect
local it = JestGlobals.it

describe("toStrictEqual string bug", function()
	it("should pass for identical strings", function()
		expect("hello").toStrictEqual("hello")
	end)

	it("should pass for tables with string values", function()
		expect({ value = "hello" }).toStrictEqual({ value = "hello" })
	end)

	it("should pass for arrays of objects with strings", function()
		expect({
			{ name = "alice" },
			{ name = "bob" },
		}).toStrictEqual({
			{ name = "alice" },
			{ name = "bob" },
		})
	end)
end)

Expected

All 3 tests pass.

Actual

All 3 tests fail with "serializes to the same string".